Part of what makes NCAA men's basketball so exciting is that it works on a rating system. They don't simply select 68 random teams to compete for the championship every year. Instead, there's a method to their madness, and different teams in different conferences earn different rankings based on a slew of metrics which includes the team's wins and losses. Also taken into account: The strength of the opponent, the strength of schedule, points scored vs. points allowed, and other deterministic metrics to rank teams from 1 to basically 100, while only the top-25 really carries any weight (particularly per division). It is these top ranked teams that garner a lot of support and attention from those who enjoy legal basketball betting on college games.
Overall, the NCAAB ratings are spread out over divisions. Brace yourself; this can start to get rather lengthy. You have conferences such as: the American East, American, Atlantic 10, ACC, Atlantic Sun, Big 12, Big East, Big Sky, Big South, Big Ten, Big West, Colonial Athletic, Conference USA, Horizon League, Ivy League, MAAC, Mid-American, MEAC, Missouri, Mountain West, Northeast, Ohio Valley, Pac-12, Patriot League, SEC, Southern, Southland, SWAC, Summit League, Sun Belt, West Coast, and WAC standings. Of all these dozens of conferences, the top contenders from each are plucked out and earn a tournament bid, until 68 spots are filled. However, the top-25 are shoo-ins; it's everyone else vying for the final spots, and playing for seeding. These rankings play a contirbuting role for those who enjoy legal NCAAB betting action. .
Take four equal squares and arrange them on a sheet of paper. Now, inside of each one of those squares, place the top 16 ranked NCAAB teams based on where they're located: South, West, East, Midwest. Each location is a category for a bracket, and each bracket is filled with that region's top 16 basketball teams, creating the stage for betting on March Madness. This is how the final tournament is seeded, and the round of 68 plays, whittling down to 34. From there, the winners move on to the Sweet 16, after which the Elite Eight play. The winners of the Elite Eight move on to the Final Four, and the winner of each of the Final Four match play in the college basketball final. It may seem a bit complicated on its face, but the ball literally starts moving along once the seedings are taken care of.
The odd thing that confuses a lot of people about NCAAB men's rankings it that a team that's #4 on the top 25 poll may actually end up being a #1 team in the tournament bracket. But this is due to the four-way split of the conferences, so remember that seeding is different than basic top-25 rankings. The top-25 is basically just a gauge of a team's success during the season. Unlike football (FBS), the ranked teams don't have to worry about vying for bowls; it's mostly about tournament seeding. There are two main top-25 polls, the AP poll, and the Coaches poll. Here is what the AP currently lists as the top-25 in men's NCAA basketball for the 2015 - 2-16 season.
8: Notre Dame
9: Iowa State
14: Wichita State
15: North Carolina
20: West Virginia
23: Michigan State
All these rankings are subject to change with every win or loss, and they will all change when it's time to rank teams for the four sections of the March Madness tournament.
You can check the latest rankings on the official NCAAB Rankings Page.