For every sport except for football the NCAA has three divisions. Division I, II and III. The differences between the three are the number of sports sponsored by the school as well as the number of scholarships offered for those sports. For example, division III has no scholarships.

 

For football, the NCAA has four divisions. Division I-FBS (Football Bowl Subdivision), Division I-FCS (Football Championship Subdivision), Division II and Division III. There are two differences between FBS and FCS schools. The first difference is that FBS schools must meet minimum attendance requirements for all of their games. The second is that FBS schools can field more scholarships for football than FCS schools can. There is also a key difference between FBS and FCS play, which is the organization of the post-season. FBS schools are governed by the BCS ranking system and use the bowl format in the post-season. FCS schools enter a tournament playoff system based on regular season conference championships and a few at-large bids, in the same manner as the Division I basketball tournaments. Division I-FBS, I-FCS and II schools may all compete against each other during the regular season, though such interaction is usually limited to 1-2 games per season for most schools.

(Quoted from: http://www.guardyourpants.com/blog/?feed=rss2) 

 

 

Found some source to explain the BCS sysytem:

 

A team's BCS ranking is based on three factors:

• standard computer ranking
• writers' poll
• coaches' poll

Each component is measured by team-by-team comparisons. For the standard ranking and writers' poll, this is based on the probability that one team is better than the other. For the coaches' poll, this is based on the fraction of coaches who placed one team ahead of the other on the ballot.

(Quoted from: http://www.dolphinsim.com/help/college/bcs.shtml)

 

Under the system, a team will have a percentage score from each of the three components. These percentages will be averaged to determine a team's BCS ranking.

When using the AP (Associated Press) poll and coaches' poll, teams will be evaluated on the number of voting points it receives -- not the individual ranking number. The AP poll has a total of 1800 possible points, while the coaches' poll has a total of 1500.

The computer ranking portion of the system will consist of six ranking systems. Those retained were Jeff Sagarin, Anderson & Hester, Richard Billingsley, Colley Matrix, Kenneth Massey and Dr. Peter Wolfe. The New York Times withdrew from the ranking system after last season.

When calculating the rankings from each computer system, the highest and lowest ranking for a team will be discarded. From the remaining four rankings, points will be assigned in inverse order from 1-25, averaged out and calculated as a percentage of 100.

For example, if a team receives 1760 AP poll points (.978), 1440 coaches' poll points (.960) and a possible computer ranking of 94 points (.940), its BCS average will be .959.

(Quoted from: http://www.wftv.com/cfoot/3532700/detail.html)