This article is from the Toyota Tundra FAQ, by Steve Yee with numerous contributions by others.
Base - Base is exactly that. It's the no-frills version of the Tundra, ususally
equipped with a V6, manual transmission, and has some decent options like Air
SR5 - This is the most flexible version. You can go as low or as high in terms
of the options as you want. Most SR5's are found either as 4x4 single cab
longbeds, 4x2 Access Cab Shortbeds, or 4x4 Access Cab Shortbeds. Most have V8
Limited - This is the top of the line. Most are Access Cabs with either 4x4 or
4x2 suspensions, V8 engines, and leather bucket seats.