(In baseball) The road team bats first ("top of the inning") and the home team bats last ("bottom of the inning"). Between that ("middle of the inning"), there's a break for the teams to exchange places (and for television to run
commercials and fans to get a
beer.)
If you really want to get technical, a regulation (ie. "normal") game is 9 innings. However, a game can be terminated earlier if it is raining/snowing, and still be official if 5 innings are completed (or if the top half of the 5th inning is completed and the home team is leading).
If a game is tied after 9 innings are complete, extra innings are played. Each team gets 3 outs (same as in the first 9 innings) and if at the end of an inning, one team leads, the game is over. In other words, innings are played until the deadlock is broken (extra innings are NOT sudden death. The home team still gets their chance regardless of if the road team scores or not.) Most extra inning games end in the 10th or 11th inning, however there have been some marathon games.
The longest in professional baseball was a 33 (yes, 33) inning affair between the Rochester Red Wings (Baltimore Orioles' minor league team) and the Pawtucket Red Sox (Boston Red Sox' minor league team) of the International League. The game started on April 18, 1981 and was suspended past 4 AM on the 19th, with the score tied 2-2 after 32 innings. The game was continued in the 33rd inning on July 23, 1981, before a regularly scheduled game. The end was quick as Pawtucket scored in the bottom of the 33rd, ending the game that took 8 hours, 25 minutes (only 18 minutes of which were in the continuation part).
Especially noteworthy about the game is that 2 future Hall of Famers were involved. Both Cal Ripken, Jr. of Rochester and Wade Boggs of Pawtucket played the entire game. In fact, Boggs doubled home a run in the bottom of the 21st to keep the game going.
The major league's longest game was May 1, 1920. It lasted "only" 26 innings, ending in a 1-1 tie between the Boston Braves and the Brooklyn Dodgers.