Above is correct - a regular mattress pad is just that - a thin pad, sometimes with waterproofing, to protect your mattress from spills, etc. Yes they do sell these to go on top of pillow-top mattresses. Just be sure to buy an extra deep one or it won't fit over it.
A mattress topper is much thicker, not to protect your mattress but for extra support or comfort while you sleep.
We just got a 3-inch mattress topper for DS11 because his mattress is older & starting to wear out. We thought we'd save money by just getting a $45 topper rather than a $200+ mattress, but he doesn't like it.