There is a good link that summarizes and explains graphically what happenned, but unfortunately it's in Russian only.

http://eco.rian.ru/ecoinfogr/20090425/169208464-ig.html

(requires flash)

There's another source (unfortunately in Russian too):
I'm inclined to believe this version:
http://www.ufo.obninsk.ru/chernob0.htm (part 1)
http://www.ufo.obninsk.ru/chernob1.htm (part 2)


There's an alternative opinion. It his book about the accident
Anatoly Dyatlov, a former chief engineer of the power plant states that there were faults in the reactor's design.
Well, he's dead already and he was the one who suffered the most, but nevertheless his arguments are weak.

Not to go too deeply into specifics I can illustrate his point like this:
Imagine, you have a TV set, and you are trying to 'experiment' with it. You pull out an electric fuse and attach the mains directly, then you open the casing and put your hand onto the leads. Naturally you will suffer an electric shock, but you will say afterwards that the fact that 'something might go wrong if you put your hand onto the leads' was nowhere in the documentation for your TV.

His book:
http://lib.ru/MEMUARY/CHERNOBYL/dyatlow.txt