An H-back is basically half tight end, half fullback, and the position is usually played in an offense that starts 2 tight ends and 1 back, such as the one the Redskins are running currently under Joe Gibbs. The H-back usually catches more passes out of the 2 TEs in such an offense, as seen in the case of Cooley, who is a pass catcher as opposed to the 'Skins other starting TE, Walter Rasby, who is basically just a blocker. Probably the most prominent H-back of recent times is the recently retired Frank Wycheck, who was Steve McNair's most frequently targeted receiver playing basically the h-back position in the Titans offense.
Hopes my explanation helps!
