Vines

Vines (also called Ivy) are non-solid blocks that can only be placed against a solid block, such as onto walls and ceilings, and can be climbed similarly to ladders. Unlike ladders, vines do not have a collision box, so the Player cannot 'walk' on top of a vine without falling; however, like ladders, it is possible to 'hang' on vines by sneaking.

Vines have the same color as other foliage in the same biome. They commonly grow on leaf and wood blocks in swamp and jungle biomes, but they will also spread onto stone, planks and most other block types. Vines take as long to break as leaves and drop nothing if broken by hand, but they can be collected using shears. Although they can be placed after collection, they can only be placed onto the side of a solid block, not on the top or bottom. Breaking a block after vines are placed on it will cause the vines to disappear unless they are still supported by other vines.

Vines are considered 'replaceable' in the same way as Grass and Fire. This means that one may place a block in the same space as a vine block, and that will cause the vine block to be instantly removed.

Like tall grass, vines are most effectively destroyed by water; vines on a tall structure can easily be washed away by placing a water source block at the top of the structure.

If the player is near a swamp or jungle biome, vines may be easier to obtain than ladders. This makes them useful as a ladder substitute, if the player has run out of ladders, does not want to use them, or particularly wants a 'green' or camouflaged look for a building design.

Vines cannot be placed directly on certain blocks, including Chests, Crafting tables and Cobwebs, but they can be grown around them.

Uses

 * Vines can be climbed, but only when they are next to a solid block. This makes them a cheap alternative to ladders for a player who doesn't have much wood on hand and doesn't have enough time to farm it. Vines are more fragile than ladders and cannot be retrieved without Shears.
 * Vines greatly reduce the chances of taking fall damage in jungle biomes since they absorb all fall damage.
 * When descending vines, if the vines overhang an empty space, it is possible to continue descending down the vines as if there were a block there. You can't climb back up the vines without solid blocks behind all of the vine pieces, though.
 * Vines block mobs' line of sight, so it is possible to sneak up onto mobs by hiding behind vines. Because of this, Endermen can safely be viewed through vines. You can also shoot through vines, but skeletons can't.
 * Vines can be used to hide other blocks or passageways such as doors or even portals in a similar way to Paintings. This allows the player to create hidden passageways or concealed blocks.
 * Sneaking on vines causes the player to hang onto them, even if the vines are not next to a block. This can be used to "hover".

Farming
Vines will grow into an open space below them. Making a bar a few meters off the ground, and then attaching a row of vines below it is the most efficient way to farm them. When harvesting hanging vines it is important to remember to work from the bottom upwards for maximum yield. Breaking a hanging vine will destroy all sections lower than it, but will only yield one piece of vine regardless of how many pieces were destroyed. Vines will grow at any light level, including in complete darkness.

Both small and large jungle trees, when grown, usually have vines growing off of them. Oak trees planted in swamps also usually have vines.

History
When the Adventure Update was showcased at PAX, vines hadn't been given a name, and a mouseover would not show a title. This implies that they didn't have an official name at the time.

Bugs

 * Multiple vines can be in the same block space, but count as one block for most purposes. See Talk for discussion
 * If you keep placing vines onto the same square, it will use up your vines but only place a single one.
 * Vines seem to have a bug where they can grow behind paintings.
 * When held and viewed through third person view or held by another player, vines have a gray texture, like Lily Pads. This is because they can be colored according to the biome. A similar effect is in the texture packs of grass, lily pads, and leaves.
 * Vines can be climbed without a block being behind them by having a block 1 block away.
 * Because of their lack of collision box, vines and ladders cannot be easily connected. Ascending from a vine to a ladder requires an adjacent surface to press against whilst climbing, and descending from a vine onto a ladder requires the player to move off the collision box.
 * If two vines are hanging from the outside corner of a block, the player cannot hang from either one of them.

Trivia

 * If blocks are placed near a vine (such as the case of having a house built in a swamp biome, for instance), vines will start growing on the wall.
 * Vines can grow in any direction when placed on a block. When there is no block next to the block where the vine is on it can grow to the side of the block. The vine can even grow up, But not if there are four other vine blocks nearby.
 * When vines grow down they don't need a block below them to attach to, however, the top-most vine needs a block in order to attach the entire vine.
 * You cannot place vines directly below blocks but they can grow under blocks.
 * When placed on a wall that is touching the bottom of a block, the vine will also attach to the bottom of the block.
 * Vines have the ability to support sand and gravel. However, unlike other such blocks, vines will be destroyed when sand or gravel attempts to occupy the space adjacent to the vines.
 * Vines make the same noise as grass when stepped on.