There's understandable growls such as in some Opeth songs, and growls that aren't understandable, just listen to some early Cannibal Corpse songs or something like that.
Growling is done by pushing air with your diaphragm. Allot of people think they can growl by just yelling or talking and then pushing together their throat, but all they're gonna do is mess up their throat and vocal chords. The only correct way is to use your diaphragm.
You can try to bark like a rottweiler right? Like a low, growling and short but powerfull *woof*. Practise that.
Now if you do that, but start *woofing* 1, 2, 3, 4, or A, B, C, D, etc you will start how to use those low *barks* for words. Another way to explain it is that sometimes when you're tired you can create a really loud and raspy sigh. It'll sound distorted and low, but it won't hurt your throat (just like the barks). That's because you use your diaphragm!
Try to control that, use your diaphragm, form words and that way you can start to learn how to growl (or 'grunt').
The highth of your adams apple and the way you shape your mouth and your tongue placement determine the highth of your gutteral vocals.
Low growling; listen to: Suffocation - Effigy of the Forgotten
High Growling/Screaming; listen to: Children of Bodom - Needled 24/7
Low screaming; listen to; Cannibal Corpse - Make Them Suffer
High screaming; listen to; Chimaira - Pure Hatred or some Nasum song.
*Undeceived* by Extol - Heavy grunting, reasonably understandable, and very cool...