Offense and Defense switch sides for 9 innings/frames.
4 bases in a diamond. You must touch all 4 bases to score a run.
If you hit it over the fence(home run) you get to touch all 4 bases.
Lines are out of play.
As simple as I could get as long as you know that there are 2 teams and when you cross home its a run. People love Home Runs, and when their pitchers give up no hits.