I'm no good at baseball, but it's pretty well done. You hold the Wii remote like a baseball bat, the buttons facing you, and the pitcher throws tha ball. You swing the Wiimote in real life and your character on-screen swings his/her bat at the same time. You can't really run the bases--however far you go depends on how far you hit the ball. When you get three outs, you get to pitch. There are ways to do advanced pitches, but the simple way is to just swing the Wiimote. The faster you swing it, the faster the ball is thrown. The Mii (customized character you made) you chose is the captain/pitcher you play as, and any other Miis you have play on your team with you. I've batted as a Lego Man and Alex Trebek. You even get neat-o striped pants.
Did you get all that? If it sounds dangerous, don't worry--the wrist strap will help protect your French vase by eliminating flying remote controls from your home.