Grass Block

A grass block is a natural block that generates abundantly across the surface of the Overworld.

Natural generation
Grass blocks generate naturally in most biomes in the Overworld and as part of villages.

Post-generation
Grass grows spontaneously only during map generation. Afterward, it can spread only to a nearby dirt block. Grass spreading without player intervention depends heavily on the time of day. For a dirt block to accept grass from a nearby grass block, the following requirements must be met: Light-reducing blocks include any opaque block, as well as lava, water, ice, and partially transparent blocks like stairs and single slabs. Grass blocks can grow under all other transparent blocks like glass, fences, or torches. Grass jumps directly from one block to a neighbor and is not affected by gaps or other blocks being "in the way." Grass blocks spread at random intervals and have an equal chance of spreading to any suitable dirt blocks that are in range. Because grass can spread as much as 3 levels downward, it tends to spread down slopes much faster than it spreads up them.
 * 1) The dirt block receiving grass must be within a 3x3x5 range of the source block where the source block is in the center of the second topmost layer of that range.
 * 2) The source block must have a light level of 9 or brighter directly above it.
 * 3) The dirt block must have a light level of at last 4 above it.
 * 4) Any block above the dirt block must not reduce light by 2 levels or more.
 * 5) It cannot spread to coarse dirt.

From block loot
A grass block can be obtained by mining it using a tool enchanted with Silk Touch.

From mob loot
Endermen can pick up grass blocks, and drop the blocks they are holding if killed.