The following is an edited excerpt from Wikipedia:

“Softball is a game similar to baseball played with a larger ball (11 to 16 inches in diameter) on a smaller field.  Softball rules vary somewhat from those of baseball. Two major differences are that the ball must be pitched underhand and that seven innings, or 1–2 hours depending on the league, constitute a regulation game compared with nine innings in baseball.

The game moves at a faster pace than traditional baseball due to the field being smaller and the bases and the fielders being closer to home plate.

In fast pitch softball the entire infield is dirt, whereas the infield in baseball is grass except at the bases and on the pitcher's mound which are dirt. Softball mounds are also flat, while baseball mounds are a small hill. Softballs are pitched underhand, but baseballs are pitched overhand.”