After the committee assigns seeds, it places the top four teams in each region to a first/second round site that is most geographically compatible to those teams, regardless of where the team might play in subsequent rounds. For example, a team from an Eastern state that is seeded second in the West region might play its first- and second-round games at a site in the Eastern part of the country and then play in the West Regional. A Regional is a tournament within a tournament. The four champions from each region comprise the Final Four.
There are eight sites for the first/second rounds. Teams are placed at these sites in groups of four, which comprises a pod, and there are two pods at each site. For each region, there are four pods [source: NCAA].
Each week, teams play at a different site. The Final Four is played at a new venue each year, but it's usually played in a major metropolitan area.
While it sounds complicated, the NCAA tournament is actually pretty simple. The tournament keeps subtracting teams until there is only one. That team is the champion of college basketball.