Gates and limiters can be used in a variety of functional and creative ways.
Limiters are an extreme form of compression. Any sound that goes over your set threshold will be cut off (eliminated).
Gates are sort of like the opposite of a limiter (although you can use a reverse gate to achieve the same effect). Gates are functionally useful when trying to get rid of background/unwanted noise. Typically, in a gate, you set your threshold so that any sound that is BELOW your threshold will be silenced. This is particularly useful when recording vocals. You don't want the sound of the room/background/ambience - you just want the singer's voice. A gate can help eliminate the ambient buzz when the singer isn't singing.
Limiters can also add volume to your track (at the expense of dynamics).
Gates can be used creatively (google trancegate), too.
Hope this helps.