Not sure if anyone has posted this but...
There are two or even three types of Running Backs. Accualy technically 2.
One is a Tail Back or Half Back. Half Backs are more of the speedy smaller backs,
Fullbacks are huge brick walls which is always good to have speed but must have the size (not hieght but weight) to put on some nasty blocks for the HB to break away and too block for a QB also. Rarley they are given the ball, but can be a great option for short yard situation. Look up Jim Brown if you want to know a great FB....but since then there has rarley been as beastly as a FB as him.
Also there are diffrent types of HBs, Recieving backs, 3rd down backs, Short yeard running backs, Break away speedy running backs, every down running backs all that great stuff.