Basically a Buffalo room is in one of the buildings that are adjacent to the main facilities, the Rio Grande rooms are all along the river and the standard rooms are everywhere else. The rooms themselves are all exactly the same it is simply the location that is different. If you get a buffalo room you may end up in a noisy place as you are by the main facilities, if you have a Rio Grande room you will be near the river but may not have a river view, if you're in a standard room you may have a tree lined view or a car park! They're very hit and miss as you don't get further choices in these categories it simply gives you a selection of buildings that you can narrow down where you will be staying!
