Mattress toppers are more substantial than mattress pads and protectors, typically around 1" to 4" in thickness. Rather than protect your mattress, they're designed purely to add an extra layer of comfort and support to new or older mattresses.
A mattress pad is a thin quilted layer designed to protect your mattress from light stains and general wear & tear.
Mattress protectors are thinner like fitted sheets. They're primarily used to protect your mattress from spills and humidity, making them a great choice for kids, homes with pets, and humid climates. They're also great at keeping contaminants like mold, bacteria, and bed bugs out of your mattress.