The NCAA Men's Division I Basketball Championship, or NCAA Tournament is a single-elimination (if you lose a game, you stop playing) college basketball tournament with 68 college teams playing each other. The National Collegiate Athletic Association (NCAA) created it in 1939. When it was started in 1939, it only had eight teams. It has grown slowly, but steadily.
The tournament is played from mid-March to early April, so basketball around that time is sometimes called "March Madness".
There are two ways to get in. You can win your conference (a group of about 8 to 12 basketball teams) championship, or get voted in by the NCAA. At first, only one team was allowed in from each conference, but now more can come from each conference.
The tournament is split into four groups called regionals. Each regional has 17 teams, seeded (numbered from best to worst) 1 through 17. The 16 and 17 seeds play each other at the beginning. Then the 1 seed plays the 16 seed, the 2 seed plays the 15 seed, and so on. Then the winners advance to the next round. There are six rounds.