Memory foam is a synthetic foam constructed with polyurethane and additional materials that create viscoelastic properties. Each manufacture of memory foam (and there are a bunch) offer their own "ingredients" and own "recipe". A memory foam mattress will offer a base foam for the structure of the mattress and atop that will be layers of contouring foam. Memory foam is known for its pressure relieving properties that hug and contour to your body. There are 3 main types of memory foam.
1. Traditional Memory Foam - May retain heat and moisture
2. Open Cell Memory Foam - Allows for more comfort & Pressure relief
3. Gel Infused Memory Foam - Allows for more comfort, and Cooling.
The more dense the foam the better feel you will get.
When you lie down and apply pressure to a memory foam mattress with your body, it will hug and contour your form. Response times (how quickly the mattress changes to pressure) vary based on the type of memory foam being used, but in general, memory foam is going to respond slower than most other foams.