Here is a really good website that answers this question for you. (it even shows pictures, wink)
http://www.kettering.edu/~drussell/bats-new/baseball-softball.html
Below is an excerpt taken from the page. I hope this helps.
At the youth level there is no difference in bats for softball and baseball. The dynamics of the game at the youth level are quite different than when played by adults who are stronger and more experienced. But, at the adult level, baseball and softball bats are designed to match their respective games. Conceivibly you could use a baseball bat to play softball, as long as you can adjust to the slightly shorter total length, and the heavier weight. You would probably find that the performance would be about the same as an older lower performance softball bat. Using a softball bat to play baseball would most likely result in cracking or breaking your bat. Softball bats are not designed to withstand the forces resulting from an impact with a baseball. The performance might be really good and the ball might come off the bat pretty fast, but you would most likely break your softball bat. The same would be true of using a fast-pitch softball bat in a slow-pitch game; you would probably damage your fast-pitch bat. You could use a slow-pitch bat in a fast-pitch softball game without damaging it but the larger weight (actually, larger moment-of-inertia) makes the slow-pitch bat more difficult to swing quickly and you would probably not hit as well.