What are the major differences between the different versions of the PS3?

Actually the only version being produced is the slim version ... you won't find a new -fat- version anymore... (unless someone sells them like diamonds). Anyways. The difference among the new models (slim ones) is based in their hard drive size. The actual versions are 120Gb and 250Gb ... but beware, this month Sony Japan will release new Slim versions in white color with higher high drives ... !!!

You can read further information on technical differences here: http://en.wikipedia.org/wiki/PS3#Model_comparison

The Playstation 3 Wikipedia article has a pretty good breakdown of all the different models and their capabilities.