There are many types of grasses so describing them would be meaningless. Considering that any person with a right mind has seen any form of grass whether it be Kentucky Bluegrass, or Crabgrass. We've all seen them. Grass was originally introduced to this region by the feces of cattle and other grain eating animals. Now we have cultivate it so that we can grow the stuff for our lawns, and it escapes regularly into wild areas.
Why Annual Grasses are Undesirable:
Annual Grasses are know to have a faster life cycle than any native plant. So they now can out compete the existing plants and in some cases cause their extinction. They come up fast and grab all the available nutrients and water. In fact, the wind can carry their seeds far from the mother plant so that they can grow wild in uninhabited areas. Some of them even have stickers so that people and animals carry them everywhere. Most california native grasses were perennial bunch grasses which grew slowly and like alkaline soil. The european grasses have changed the chemistry of the soil to be more acidic, making many areas undesirable for native grasses.