An actuator is just something which causes movement.
A valve actuator is one which operates a valve, for instance you could use them in a simple hydraulic circuit to effect a valve to direct flow into a cylinder or motor.
A rotary actuator is one whose motion is in a rotary manner. A steering actuator could be used to effect a steering mechanism. It's all pretty self explanatory.
Back to your original questions, do you know of any formulae which relate force and pressure? Looking at the units of these factors might help you out. But cristo is correct, nobody is going to do your homework for you.