There are different versions of softball: slow-pitch and fastball. Slow-pitch uses a looping pitching delivery, and a fourth outfielder/rover. It's less competitive. Fastball uses a "windmill" underhand delivery, which is actually the fastest way to deliver a ball. The best fastball pitchers can hit 105+ mph, better than a baseball pitcher, due to the longer period of delivery (more rotation.) In both cases, the ball is about 2x as large as the regular baseball.
Baseball, or hard-ball, uses the smaller baseball, and an overhand delivery. It is the "typical" game seen on TV played by the professionals.
There are many other differences in rules between the 3, as well as the size of the field, and base distances, but those are the main visible differences.
I used to play on a competitive fastball team, infield, catcher, and relief pitcher.