Grass is a very common resource in Mine Blocks. It is located at the surface of the world and can grow on dirt if it has nothing above it. It can be easily acquired by punching it. When mined, it drops dirt. Tilling grass with a hoe will turn it into farmland.
All passive mobs can only spawn on grass or dirt blocks. If you want to make an animal farm, make sure you place dirt or grass as the floor.
Grass and dirt