The tournament was created in 1939 by the National Association of Basketball Coaches, and was the idea of Ohio State coach Harold Olsen. Played mostly during March, it has become one of the most famous annual sporting events in the United States.
It has become extremely common in popular culture to predict the outcomes of each game, even among non-sports fans; it is estimated that tens of millions of Americans participate in a bracket pool contest every year. Mainstream media outlets such as ESPN, CBS Sports and Fox Sports host tournaments online where contestants can enter for free.
The tournament teams include champions from 32 Division I conferences (which receive automatic bids), and 36 teams that are awarded at-large berths. These "at-large" teams are chosen by an NCAA selection committee, then announced in a nationally televised event on Sunday preceding the "First Four" play-in games, currently held in Dayton, Ohio, and dubbed Selection Sunday. The 68 teams are divided into four regions and organized into a single-elimination "bracket", which pre-determines, when a team wins a game, which team it will face next.
Each team is "seeded", or ranked, within its region from 1 to 16. After the First Four, the tournament occurs during the course of three weekends, at pre-selected neutral sites across the United States. Teams, seeded by rank, proceed through a single-game elimination bracket beginning with a "first four" consisting of 8 low-seeded teams playing in 4 games for a position in the first round the Tuesday and Wednesday before the first round begins, a first-round consisting of 64 teams playing in 32 games over the course of a week, the "Sweet Sixteen" and "Elite Eight" rounds the next week and weekend, respectively, and – for the last weekend of the tournament – the "Final Four" round.
The Final Four is usually played during the first weekend of April. These four teams, one from each region (East, South, Midwest, and West), compete in a preselected location for the national championship.
According to en.wikipedia