Biome/Before 1.18

Sub-Biomes are hillier or flatter versions of some biomes. However in 1.18 most of the biomes were no longer needed because of the new world generator, so they were removed. Only the Savanna Plateau remained.

Badlands Plateau
The badlands plateau features large flat-topped hills composed of stratified colors of terracotta ranging in warm colors. Plateaus have steep edges that rise to within 20-30 blocks above sea level, where they quickly flatten. The top of these plateaus typically have scattered dead bushes. Occasional ponds can appear on plateau tops. The sides of the plateau can occasionally reveal caverns and mineshafts. River biomes that pass through badlands plateau biomes cut steep grooves, giving the appearance of narrow canyons. These can pose a fall damage hazard if the player is not careful. Ravines also frequently spawn in badlands plateau biomes, which cause the same as above. This biome is not always present in the badlands biomes, but it's likely to appear.

Badlands plateaus use the same mob spawning chances as badlands.

Bamboo jungle hills
The bamboo jungle hills variant is similar to the bamboo jungle, though with steeper terrain just like the regular jungle hills variant. Large amounts of bamboo cover the landscape, and patches of podzol replace most grass blocks. Naturally-generated trees are always large variants, pandas may spawn here, like in the bamboo jungle. Jungle pyramids may also spawn here $$.

$$, bamboo jungle hills use the same mob spawning chances as jungle hills for hostile and ambient categories. As for the others:

Birch Forest Hills
Birch Forest Hills features hillier terrain than regular birch forests, being identical to them in every other aspect. It is fairly common due to its wide spread.

Birch forest hills use the same mob spawning chances as birch forests.

Dark forest hills
Dark forest hills can break the leaf canopy, increasing visibility and decreasing the chance of daytime hostile mob spawning, though the hills are steep compared to other hill biomes. Hills generated near rivers can lead to cliffs, risking fall damage if players don't land in the water. Small plains-biome clearings do not generate within the dark forest hills variant. Woodland mansions may still generate here.

Dark forest hills use the same mob spawning chances as dark forests.

Desert hills
Desert hills variants feature hillier terrain, just like all other hills biomes in the game. Desert hills can reach slightly higher elevations than other hills, and are comprised mostly of sand and sandstone like the rest of the desert. No structures other than fossils, desert pyramids, and desert wells may generate within the hills, making this variant overall more difficult. Players should be careful digging up sand in the hills, as it may generate unsupported and cause all other unsupported sand blocks near it to fall if dug up. Desert hills do not generate if their base desert is a thin border around a badlands biome.

Desert hills use the same mob spawning chances as deserts.

Desert lakes
The rare Desert lakes variant features slightly rougher and hillier terrain than the base desert biome, though not as much as the desert hills. This makes Desert Lakes more likely to have oases of water across its landscape, making this variant a potentially valuable source of water. No structures other than fossils and desert wells may generate here.

Desert lakes use the same mob spawning chances as deserts.

Giant Spruce Taiga hills
Giant spruce taiga hills is a variant intended to be a more mountainous version of the giant spruce taiga. However, $$, due to a likely error in the way terrain height is calculated, there is no difference in the terrain between giant spruce taiga and giant spruce taiga hills. Specifically, the game uses internal values known as  and   when generating hills biomes, but these values are the same for both giant spruce taiga and giant spruce taiga hills, resulting in no actual difference between the two. This is the only hills biome in the game with this issue.

$$, this biome generates as a hillier version of the giant spruce taiga, however, this biome generates the same trees as giant tree taiga hills tree type (not giant spruce tree type) resulting in no actual difference between giant tree taiga hills and giant spruce taiga hills (except in water color).

Giant spruce taiga hills use the same mob spawning chances as giant spruce taigas.

Giant Tree Taiga Hills
Like all other hills biomes, giant tree taiga hills feature elevated, hillier terrain compared to the normal giant tree taiga, making the landscape less suitable for shelter. Podzol, coarse dirt, and rocks all still generate on the hills. Wolves, foxes and rabbit spawn here.

Giant tree taiga hills use the same mob spawning chances as giant tree taigas.

Gravelly Mountains+
Gravelly mountains+, also referred to as modified gravelly mountains in code, is a rare variant of the wooded hill biome that has the exact same features as the regular gravelly mountains, making this biome almost indistinct from the former. with the only difference being the fact that it can rarely generate standalone as a thick separation when a desert lakes biome borders a snowy biome.

Gravelly mountains+ has the same mob spawning chances as mountains.

Jungle hills
Similar to the wooded hills biome, the jungle hills biome features steeper terrain, making it a more difficult variant of the already difficult jungle for survival purposes. Ocelots, parrots, and pandas spawn here and jungle pyramids may still generate here.

$$, jungle hills use the same mob spawning chances as jungles for ambient categories. As for the others:

$$, jungle hills use the same mob spawning chances as jungles.

Modified Badlands Plateau
The modified badlands plateau features smaller plateaus and somewhat harsher terrain than the Badlands Plateau, mimicking large plateaus that have weathered more over time. Eroded badlands replace the usual thin Desert border this biome variant would share with other biomes. The Modified Badlands Plateau is the second rarest biome in Minecraft, after Modified Jungle Edge, and is only present in about 1/5 of the badlands biomes, and almost always (98% chance) comes with an eroded badlands bordering the edges and modified wooded badlands plateaus surrounding it at the center.

Modified badlands plateaus use the same mob spawning chances as badlands.

Modified jungle
The rare modified jungle variant features much more mountainous terrain, being taller and steeper than jungle hills. The heights, combined with the thick foliage, renders the ground below almost entirely out of sight. Ocelots, parrots, and pandas may spawn in this biome, but jungle pyramids do not generate here, modified jungle biomes are similar to the jungle hills, but can sometimes have differences.

$$, modified jungles use the same mob spawning chances as jungle hills for hostile and ambient categories. As for the others:

$$, modified jungles use the same mob spawning chances as jungles.

Modified jungle edge
The rare modified jungle edge variant generates only in strict conditions, and is currently the rarest biome in the game. If a jungle biome borders a swamp hills biome, then the modified jungle edge spawns as part of a double-layered transition, with a thin normal jungle edge bordering the swamp hills, and the modified jungle edge bordering the jungle. As both jungles and swamp hills are already rare, and even more rarely do they generate bordering each other, the conditions for a modified jungle edge to generate are rarely met. When they actually do manage to generate, they are often just a few hundred blocks in length, but in some cases are less than 10 blocks, making them one of the smallest biomes as well. Modified jungle edges feature the same smooth transition and lowered tree density that regular jungle edges have, though with much more mountainous terrain and occasional overhangs. Ocelots, parrots, and pandas may spawn in this biome, but jungle pyramids do not generate here. Modified jungle edges cover only a few millionths (0.00027%) of the overworld by area. For comparison, the rare mushroom fields biome covers about 0.056%.

Modified jungle edges use the same mob spawning chances as jungle edges.

Modified Wooded Badlands Plateau
Similar to the modified badlands plateau, the modified wooded badlands plateau has a weathered appearance and features smaller plateaus with more erratic terrain, allowing for significantly fewer oak trees to grow at the highest layers. Eroded badlands replace the usual thin Desert border this biome variant would share with other biomes, the biome is present in 1/3 of all badlands biomes and an eroded badlands is nearly always (98%) present at the edges of this biome.

Modified wooded badlands plateaus use the same mob spawning chances as badlands.

Mountain Edge
The mountain edge variant used to generate before Java Edition 1.7.2. It is currently unused and it will be fully removed. Similarly to the jungle edge biome, it was a technical biome intended to provide a smooth transition from other biomes to the mountains. It is similar to the wooded mountain biome, but with a few oak trees. The terrain was gentler and not as steep as the normal mountains, with oak and spruce trees growing.

Mountain edges have the same mob spawning chances as mountains.

Mushroom Field Shore
The mushroom field shore is a technical biome, that represents both the shores and the rivers of the mushroom fields. It generates when a river cuts through it as well as when it borders an ocean, unless the ocean is a deep variant, in which case a steep cliff generates instead. The terrain of this biome is much flatter and shallower in elevation, similar to beaches, though it is equal to the mushroom fields in every other way. Buried treasure can generate here and sometimes shipwrecks.

Mushroom field shores use the same mob spawning chances as mushroom fields.

Savanna Plateau
The Savanna Plateau is similar to the hills variants of other biomes, being a gradual rise in terrain, though flattens on the top about 20–30 blocks above sea level. The top of the plateau is similar to the regular savanna, featuring acacia trees and tall grass, though villages do not generate here. Horses still spawn on the plateaus and this is the only savanna sub-biome where llamas spawn.

$$, savanna plateaus use the same mob spawning chances as savannas for ambient and hostile categories. As for the others:

$$, savanna plateaus use the same mob spawning chances as savannas.

Shattered Savanna Plateau
Like the normal shattered savanna, the shattered savanna plateau variant featured steep mountains, cliffs, and overhangs, which made it a treacherous place to explore. Though it was nearly indistinguishable from the regular shattered savanna at first glance, the shattered plateau's terrain was slightly gentler, though often risked fatal fall damage if not above water. The giant lakes characteristic of the regular shattered savanna did not generate here either. $$, the foliage was a more vibrant green color, and rain would often occur in it.

Shattered savanna plateaus used the same mob spawning chances as savannas.

Snowy Mountains
These hills are no taller than most other hill biomes in the game, despite the name 'mountains'. No structures generate in this biome, though polar bears, rabbits and strays still spawn. Caves frequently generate on the sides of the mountains. $$, no hostile mobs other than strays and skeletons may naturally spawn here.

Snowy mountains use the same mob spawning chances as snowy tundras.

Snowy Taiga Hills
Like all other hills biomes, snowy taiga hills feature hillier, more erratic terrain. These hills are somewhat steep, making this variant difficult for survival mode. Pillager outposts and villages can generate in this biome, however, unlike the regular snowy taiga, igloos cannot generate here.

Snowy taiga hills use the same mob spawning chances as snowy taigas.

Snowy Taiga Mountains
The very rare snowy taiga mountains features much steeper terrain than the hills. Similarly to the taiga mountains, this variant can reach high elevations. The steep elevations make this biome difficult for survival. Buildings do not generate here. This biome is the third rarest in the game, behind modified badlands plateau and modified jungle edge.

Snowy taiga mountains use the same mob spawning chances as snowy taigas.

Swamp hills
The swamp hills variant featured hillier terrain rising up between the flat marshes. These hills would tower over the otherwise low-elevation swamp. Additionally, flooded areas in swamp hills tended to reach lower depths than the rest of the swamp, sometimes deep enough to have a gravel floor in place of a dirt floor, like normal oceans. Swamp huts did not generate in swamp hills, nor did slimes spawn, but fossils did still generate underground. Additionally, seagrass did not generate in flooded areas of swamp hills. If it connected to a sparse jungle it had a chance to create a modified jungle edge biome.

Swamp hills used the same mob spawning chances as swamps.

Taiga Hills
Taiga Hills, like all other hills biomes in the game, feature steeper terrain compared to the base taiga biome. Villages and outposts do not generate in this biome, though wolves and foxes still spawn.

Taiga hills use the same mob spawning chances as taigas.

Taiga Mountains
The rare Taiga Mountains variant are much steeper than the taiga hills, with peaks occasionally crossing the snowfall line. The steep terrain makes this a more difficult version of the regular taiga. Like the hills, villages and outposts do not generate here, though wolves and foxes still spawn.

Taiga mountains use the same mob spawning chances as taigas.

Tall Birch Hills
Like the other hills biomes, the Tall Birch Hills biome has hillier, rougher terrain, along with the taller-than-normal birch trees of the Tall Birch Forest variant. The hills are steep in this biome, comparable to the mountains biome.

Tall birch hills use the same mob spawning chances as birch forests.

Wooded hills
Wooded hills are similar to forests, though the terrain is hillier and generally more erratic, making it less suitable for shelter. Wolves may still spawn in the hills.

Wooded hills use the same mob spawning chances as forests.

Trivia

 * For some reason Savanna Plateaus can still generate naturaly in 1.18, despite being nearly identical to the Savanna.