According to one football playbook, a U-Back is defined as: "Utility back - a second Tight End used in a one-back set." I believe Norbert is also correct when he says the H-back is generally more of a power back used on passing and running plays. Fullbacks are frequently the candidate of choice for the H-Back position since they also have to be good blockers in the backfield and are generally bigger than your pure speed running back,
i.e. halfback/tailback.
The above shouldn't diminish the value of the Tight End (U-Back) as a power blocker, either. The TE isn't just a pass receiver; he's also required to block. Another reason why some teams will bring in three (3) TEs on short yardage plays --- especially in goal line situations.
Hope this helps.
