Lawn is a piece of land which is covered with grass. Grass is the actual plant.

A lawn is an area of ground (such as the ground around a house or in a garden or park) that is covered with short grass. And grass is just grass ))

lawn is like artificial smile ....and grass is real smile which comes from heart.......

I agree with everyone else, and also: Lawn is typically owned by someone, whereas 'grass' is more generic and refers to the plant itself.