Sleep products are available as either mattresses or toppers. Memory foam is not sufficient by itself to support your body weight. That is why it is normally sold as a layer that is molded on top of a support foam layer. This is a memory foam mattress. An alternative is to provide an existing mattress as a support layer and to use a layer of memory foam on the existing mattress. This is a topper.

So, a memory foam mattress is a self contained mattress providing both support and the conforming memory foam top layer whereas a topper requires an external support layer (such as an existing mattress) and only provides the conforming top layer.