Although the definition for thrust vectoring for real aircraft is a bit more complicated, for RC aircraft the term thrust vectoring is generally used to describe a method of changing the direction of the aircraft by applying more or less power to a pair of (usually) wing-mounted motors. Applying less power to the left motor causes the aircraft to turn to the left. Less power to the right motor sends the aircraft to the right.
Differential thrust is more or less the same thing (and probably a more accurate term for most RC aircraft) -- applying differing amounts of power so that you get differing amounts of thrust from each motor. It may be found with rear-facing or forward-facing twin props.
This method of turning is often used in small RC aircraft without elevator or rudder control. For craft without elevator control, equal amounts of increasing power cause the craft to speed up (propeller spins faster) and go up, less power slows it down. The differing amounts of power act like a rudder.
