Basically: you pick a tribe, you pick a commander, you choose wut units you use, and battle for your clan. No building, just upgrading your units and battle over an area...
Like I said it's free, but you can also go paying.
Usually the paying members go for beast type because then you have more options.
I have played it till lvl 40 or so... maybe I gonna play it again...
Might wanna check it out:
http://www.sgalaxy.com/