Never played rounders. Here's as brief as a baseball fan can possibly go.
The defensive team puts 9 players out in the field (normally 3 outfielders, 4 infielders, a pitcher, and a catcher). The pitcher throws to home plate, where a batter for the other team will try to hit that ball in play. If he misses, it's a strike. If he doesn't swing but the ref says it's a good pitch, it's a strike. If he doesn't swing, and the ref says it's not a good pitch, it's a ball. A foul ball (a ball not put between the white lines) is a strike, but a batter can not foul out. If a batter gets three strikes, he's out. If he gets four balls, he "walks" and automatically gets 1st base.
The runners will try to go all the way around the four bases, back to home where they started. That counts as a run. The team with the most runs wins.
If the defensive team catches a ball hit in the air, it's an out. There's a multitude of other ways to get a runner out, too many to divulge into. Basically, if the runner is tagged while he's not on a base, he's out. If he is being forced to go to another base (eg. the runner is on first when the ball is put in play, the batter must take first, the runner is being FORCED to go to second) all the fielding team has to do is touch the base while in posession of the ball.
That's enough I guess. Hope that's basic enough.
HAVE FUN (You will)! Baseball could always need a new fan!
P.S. If (and when) you become a baseball fan, just remember a Braves fan planted the seed. You should join us Braves fans!
