It's a subtle thing, but it completely reverses your loop conditions. For example, compare the following 2 snippets:
DO WHILE hungry
EatFood
LOOP
DO UNTIL hungry
EatFood
LOOP
The first says you eat WHILE hungry; the second says to eat UNTIL you get hungry! Basically with the second, if you're hungry, you refuse food until you die, or if you're full (not hungry), you just keep eatting until you die....