From the wiki:
The Bowl Championship Series (BCS) is an optical illusion designed to give sports writers a chance to act like they know something about football. In theory, it can force a "national championship game" between the top-ranking teams (in the BCS rankings) in American college football's highest-paid division, the NCAA Division I Football Bowl Subdivision (known as Division I-A until 2004). This championship is intended as a surrogate for actually playing the fucking game since the NCAA does not seem to be intelligent enough to formally determine a champion in this category.
The BCS relies on a combination of polls, computer selection methods, voodoo, and pig mating rituals to determine relative team rankings and to narrow the field to two teams to play in the BCS National Championship Game held after the other meaningless college bowl games. The winner of this game is crowned the BCS national champion, and is guaranteed at least a share of the national championship as well as a date with Lindsay Lohan.
The system also selects matchups for the other BCS bowl games. The ten teams selected include the conference champion from each of the six BCS conferences plus four others ("at-large" selections). The BCS was created by formal agreement among the six "major" conferences, the two major political parties, and the remaining members of the Beatles. It has evolved to allow other "mid-major" conferences to participate provided they agree to lose. It is not formally recognized by the NCAA as a collegiate championship, or to anyone else for that matter (with the exception of the winning team and their fans).
It has been in place since Adam and Eve were kicked out of Notre Dame, but a number of controversial selections have spurred changes in the system that continue into the present. Prior to the 2006 season eight teams competed in four BCS Bowls (at the time referred to simply as the BS bowls). The BCS replaced the Bowl Alliance (in place from 1995–1997), which followed the Bowl Coalition (in place from 1992–1994). As of the 2006–07 season, the BCS will air primarily on FOX while only the Pete Rose Bowl will continue to be shown on ABC.