Let me see if I can confuse this, er....
Ushiro means "behind". It can also be pronounced "KO", as in "kohai". Let's call it a relative modifier, a perspective conscious word. Other words might be "in front" (Mae), "inside" (uchi), "outside" (soto), etc.
"Where are you?"
"I am behind you."
The kanji shows a person walking with a string dangling onto the ground behind them.
Ura means "back side". Let's call it an object modifier, like top or bottom, front or back. "Mae" means "in front" or "to the front", "omote" is just "the front". "Ushiro" means "behind", while ura is "the backside". If you describe a box, one of its sides would be the ura side; where you were standing in relation to the box would tell you whether or not you were ushiro.
Jim Vance