As in a heads-up tournament, or just the heads-up section at the end of a tournament?
In either case, the difference is the same: blinds rising into a fixed amount of money on the table.
Watching something like the National Heads Up Championship is a perfect example: some of the best players in the world are in the game, but once the blinds get high they're crapshooting with king-rag like everybody else.
The difference between a dedicated heads-up tournament (like the NHUC) and the heads-up section at the end of a normal tournament is, of course, that at the end of a normal tournament heads up often starts with blinds in that situation.
A cash game, on the other hand, has fixed blinds and players have the choice of playing with whatever stack size they're comfortable with. They don't have to play for stacks every single hand, but if they choose to they know they can reload if it doesn't come off.
Kinda what you were after?