The grasslands biome is filled with trees, plants, forests and other natural life. They contain many hills, mountains, and sometimes ponds and lakes. Grasslands are filled with NPCs, hostile and passive mobs, along with many plants used for taming animals for pets. These plants can also be used for potions and food to restore your health.
When you create a new map, you will always spawn in a grasslands, with a town nearby, no matter the seed.