It goes something like this...
In order to be a pilot in the AF you need to first get commisioned as an officer. To do that you need to go through one of the commisioning sources (AF Academy, ROTC, OTS). Lets say you go through ROTC. At some point you will have to make your desire known to get an Undergraduate Pilot Training (UPT) slot. EVery AF pilot (even if you already have civilian licenses) goes through AF UPT. If you are competitive enough to get a slot then after you are commisioned as a Lt from ROTC you will go to UPT at one of the training bases. Currently Columbus, MS, Del Rio, TX, and Enid, OK are the training bases. Some people actually do part of their training with the Navy in Pensacola but we'll keep it simple for now.
UPT is 52 weeks long. You will start off on the T-6 (T-37s are being phased out). Everyone flies that for 6 months and then you do what we call "track select". This basically determines who is going fighters, heavies, helos, and C-130s. You'll rank the T-38 (fighter trainer), T-1 (heavy trainer), T-44 (C-130 trainer), and the helo and based on how well you did in T-6s and how well your IPs like you you will get assigned one of those aircraft. Classes are normally 20-25 students. Most of the people get T-1s, 1 or 2 will get T-44s (C-130), 5-8 will get T-38s, and 1 maybe two will get the helo.
Lets say you get T-1s. You finish up the last half of the 52 weeks flying the T-1. Near the end of training you will get a list of all the available airplane assignments for YOUR particular class. This is called the "drop". For the most part the same types of planes are available to every class but the numbers and the bases they are going to vary. After you get the drop you will rank all of the available assigments on your dream sheet/wish list and turn it in. Once again your class standing will determine what you get. For heavy guys you could end up with a C-17, C-5, KC-10, KC-135, C-130, E-3, C-21 (I think I got them all). From there you graduate UPT and go to 3-4 months of training for your specific airplane. Now that you are a rated pilot in the AF you can work all kind of magic. You are pretty much stuck in the heavy community but plenty of people bounce from one airplane to the other (at least once). There is always the opportunity to go back to UPT to be an instructor too...
Hope this helps...