Click this: http://callofdutyzombies.com/forum/viewtopic.php?f=42&t=930 the initial thread about [brains] .
It is basicly just reputation that you can give to people who post good posts. You can give it to people by clicking the green thumb up in the top right hand corner of the post. Each click gives 10 brains although you can only do it once every few hours or something so only give it to posts you think are worthy.
Zombification (green boxes) corresponds to the amount of brains you have. It goes, in order, 'Human' , 'Almost zombie', 'Zombie', 'Undead Master', 'Epic Zombie' and 'Zombie Lord' as you can see in the link I gave you.
That should be enough to give you an understanding.
Hope it helps!
PS. With your other quesitons it may be a good idea to look in the Zombie training facility or research centre. Well that's is what I would do.