Memory Foam or visco elastic is a polyurethane with added chemicals to make it more elastic and viscose. As body heat is transferred to the memory foam it becomes more plyable making it more "mouldable" to the body.
The memory foam in a mattresses can come in various thicknesses ranging from 2.5cm - 9cm. The deeper the memory foam the softer and more luxurious the mattress can feel.
The Visco elastic foam layer can sit on top of two different support layers :
1. Springs : Pocket springs or open coil springs - this gives the mattress a springier or more lively feel.
2. HD or Reflex foam - this tends to give the mattress a more "dead" feel.
The title "Memory foam" can be slightly misleading, as the mattress will not remember the shape of your body, only that it will return back to the shape from which it started.