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.