I want to build quad-copter real soon and investigating list of materials I require to build one. Recently I came across flight controller concept which is main component of any multi-rotor. I am confused between the versions/types etc. they come in. I can see there are KK 2.1 and KK 5.5 controller available along with many more like with lcd display on it, multiwii , DJI NAZA, hobbyking etc.
I am trying to understand each controllers capabilities and which will fit to which multi-rotor setup?

Please pardon me if this topic is already covered on forum, I searched on google but not getting consolidated details for these controllers.

for a beginner kk2.1 is the best choice

As Rohan has pointed out, the KK board is cheap and east to setup. If money isn't a problem, then for the beginner the Naza is a good choice.