Grass

Grass (commonly named Tall Grass or Long Grass) spawns on grass blocks in certain biomes. Grass is not a solid block, but rather an aesthetic plant that does not obstruct movement, similar to Sugar Cane, Dead Bushes and Wheat. It occurs naturally and in larger quantities than the previous plants. Bone Meal can be used on a grass block to grow grass and occasionally flowers on it and in its vicinity.

Grass has five variants.
 * Grass (the most common)
 * A triangular-looking fern
 * A dead bush-looking plant that does not occur naturally. (not to be confused with the Dead Bush appearing in desert biomes)
 * Double Tallgrass: A two block tall grass. Can be formed by using bonemeal on tall grass.
 * Double Fern: Similar in appearance in the fern, only two blocks tall. Drop two ferns if broken with shears.
 * A recolored greenish version of the Shrub called 'Grass', used as a placeholder for the data values from 3 to 15.

All five variants will occasionally drop seeds when broken; a hoe is not required to produce seeds from this block.

Ferns occur naturally only in Jungle, Taiga, and Mega Taiga biomes. Unlike grass, they will not grow when a player uses bonemeal on a grass block. Ferns can be placed in flower pots.

The non-naturally occurring dead shrub-looking variant is not the same as the dead bush. The real dead bush will generate naturally in desert and mesa biomes, and will not produce any resources when broken. The grass variant is not naturally-occurring and even though it looks like a dead bush, it cannot be placed on sand, and will drop seeds like the other variants of grass.

Water will destroy any tall grass (as well as vines) in its way, so one can easily clear large areas covered in grass by using a bucket to place water, then pick it back up once all the tall grass in the vicinity has been destroyed. This can also be used for gathering seeds.

Uses
Harvesting grass is the primary way of obtaining seeds, the secondary way being to harvest fully grown wheat. Because wheat drops many more seeds, tall grass becomes significantly less useful as your wheat farm expands.

Using Bone Meal on a grass block will generate tall grass (and flowers) on nearby grass blocks.

Grass survives in block lighting 8 or higher. Although it may seem to survive in 7 or lower, a block update next to the grass will cause it to break.

Trivia

 * Grass cannot be directly set on fire from lightning. However, fire from nearby blocks can spread to it. Oddly enough, the top of grass can be lit but it will not burn this way. When any kind of tall grass is set on fire, the fire seems to hover just over the grass, as if it were a complete block (it technically is a complete block).
 * Even though ferns do not produce seeds in real life (ferns use spores instead), they can drop seeds if broken.
 * The color of grass is biome dependent and is usually the same color as leaves.
 * Grass is slightly offset from the block it is sitting on (as seen in the third screenshot). Jeb has said this is probably intentional to give it a more "wild" appearance.
 * A "tall grass" mod existed long before the official release of tall grass, however, the tall grass generation code was not ported to the official game.
 * Grass can appear in caves on dirt near lava, and rarely, within a few blocks of bedrock or in a village house.
 * In SMP, using bonemeal on grass in a spawn-protected area will still produce tall grass and flowers (Although there is no way to remove them afterwards)
 * Tall grass can be found on dirt blocks as well as grass blocks due to a bug in the terrain generation. This includes underground dirt, with no available sunlight.
 * Grass does not re-grow naturally. Bone Meal only grows regular grass, no matter in which biome.  Therefore, only regular grass is renewable.
 * On the Xbox 360 Edition, the dead shrub variant of tall grass is available in the creative menu.
 * On the Xbox 360 Edition, Grass is called Grass Block (not to be confused with the Grass Block).
 * If you give yourself grass with a damage value 3 and above, you will receive plant with the same texture of dead bush and named simply "Grass". The plant will be greenish-brown because its color is affected by the biome, in the inventory, it is simply green.
 * In Pocket Edition when you hack in tall grass and place it it'll be a dead bush.
 * Ferns in real life usally grow multiple stems from one plant unlike the minecraft fern where it is just one stem. It sort of resembles a small sapling.

Bugs

 * Tall grass can spawn on sand in the "Overworld" superflat preset.