Tri or quads ?

I read in many places that the tris were to be absolutely avoided, and that quads were the only possible poly. (I make what I read a bit stronger. ^^)

I just wonder why. Could anyone tell me why the quands are to be prefered to tris ?

If you make a cylinder and try to sub-surf it you’ll now why!
Tris is note very frendly to you when you are working with smoth models.
(If you don’t want to do it yourself, I made a render for you)
http://img35.imageshack.us/img35/9161/m9pistol1oe.jpg