First there is the establishment bracket, with former Florida Gov. Jeb Bush, New Jersey Gov. Chris Christie, and possibly former Massachusetts Gov. Mitt Romney competing for that semifinal slot. Former Hewlett Packard CEO Carly Fiorina would likely fit into this group. GOP nominees traditionally come from this bracket.
Then there is the conservative governor/former governor slot—with, potentially, Ohio Gov. John Kasich, Indiana Gov. Mike Pence, former Texas Gov. Rick Perry, Michigan Gov. Rick Snyder, and Wisconsin Gov. Scott Walker competing, all seeking to be non-Washington and non-Congress candidates, but each with more conservative, or at least better conservative, credentials than Bush, Christie, or Romney. In this anti-Washington environment, being able to say that you effectively governed, in contrast with Congress and Washington, certainly has some advantages among the non-purist conservatives.
In the third bracket are the more identifiably tea-party candidates, principally Sens. Ted Cruz of Texas and Rand Paul of Kentucky, but also former Rep. Michele Bachmann of Minnesota, possibly former 2008 vice presidential nominee and former Alaska Gov. Sarah Palin and even real-estate mogul Donald Trump (though both Palin and Trump are unlikely to make it past the first lap if they end up entering at all). This bracket is for the "mad as hell and not going to take it anymore" conservatives.
Finally, there is the social, cultural, and religious conservative bracket, made up primarily of former Arkansas Gov. Mike Huckabee and former Sen. Rick Santorum of Pennsylvania, tapping into the same feelings as the third group but with a distinctly moral dimension.