What is the Difference Between Baseball and Men's Softball?

What are the major differences and rule changes between baseball and men's softball?

Other than the difference in the ball itself being bigger and less dense, there are major differences.

Softball has an extra player in the lineup totaling 10.  Defensively this means there are 4 outfielders instead of 3, or once in a while that extra player will be added to the infield directly behind the bag at 2nd.

The pitching is the major difference from hardball.  Pitchers in softball have a requirement of an arc above the batters waist, no higher than 10 to 12 feet depending on the league.  Because the pitch is underhand, the scoring is much higher on average than hardball, where 20 to 30 runs can be scored per game.

A few other minor differences include no leading off as a base runner.  After the pitch a base runner can be 'picked off' through the use of a force-play (not having to tag that runner).  Also, because the pitching is underhand, most leagues have a home run rule where team A cannot have hit more than 1 more home run than team B.

These rules can vary from league to league, but to the hardcore baseball fan, these rule changes are major.