A mattress topper is an item which can be placed on the mattress to give it an almost extension, that is some extra cushioning, height, or depth. It instantly changes the overall feel of your bed and will often make it seem like new again.
A mattress protector does not change the consistency of your bed in any way, shape, or form but instead works tirelessly to protect the mattress underneath.
The difference is that one changes the feel of the bed and one protects the bed. A mattress topper will not preserve your mattress underneath and will still need to be used with a mattress protector if you want to perverse your mattresses lifespan.