I agree with GEmatt. Also you can say that "there was a beautiful lawn in the garden" referring to an area of grass that is mowed and taken care of, but you can't refer to it as "a beautiful grass in th garden"
A forest is a lot of trees.
A lawn is a lot of grass.
It's not quite as simple as that, because in some contexts either lawn or grass may be used.
Cut the grass.
Cut the lawn.
Well, I'm not entirely comfortable with cut the lawn.
Mow the lawn - better.
Mow the grass - sounds really strange.