Vines

Vines (also called Ivy) are non-solid blocks, $1/16$ of a block in thickness, 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. However, Vines will not spread onto glass blocks, however they can grow as to 'hang down' in front of and next to the glass block if validly placed above them. 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 can be placed directly on Chests and Crafting tables by placing them while sneaking. They can not be placed on Cobwebs, though.

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 them. Oak trees planted in swamps also usually have vines.

Bone Meal cannot be used to grow 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.

Trivia

 * When trying to fly through vines, you will be slowed down considerably.
 * 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.
 * If a vine is hanging (touching the ground) you can't farm right there, as if it was a full block there. So when farming in a swamp or jungle biome, be careful when you are doing rows as then you will have to either cut down the tree or destroy the vines from the source (top block of them).
 * If a vine-covered wall or tree is in an animal pasture, the animals will repeatedly climb the vines by accident, and die of fall damage or escape their pen. You will need to either destroy all the vines, or place some kind of collar on the tree to keep the vines from touching the ground.
 * Vines will grow over Redstone lamps but will not reduce their light level.
 * Naturally generated jungle trees of both sizes will have vines on their trunks and canopy edges. However, player-grown 2x2 jungle trees will only generate vines on their trunks, and 1x1 trees will have no vines at all.
 * It is possible to scale a 1x1 pillar with no vines on the bottom or second-lowest blocks, but vines on all other blocks up.