Well, putting it in really basic terms... You have two teams, one of which is in the in and outfield, while the other bats. The batting team has its players take turns hitting the ball and then, if they're successful (if not, it would be strike), running the bases and trying to make it home so they get points. Meanwhile, the opposing team tries to get them out by catching the ball or tagging the bases with it. After the batters have three outs, the two teams swap positions.

...There's a lot more to it than that, but I guess that's basically it.