Memory foam is called a visco-elastic polyurethane foam and is made of polyurethane and some other chemicals that give it density and viscosity. Memory foam is used as padding and mattresses. It reacts to heat quickly molding around the body providing even firm pressure.
Memory foam was developed in 1966 by a company working under contract for NASA to improve padding in aircraft cushioning. Initially the memory foam was used in medical situations and for safety equipment such as padding in football helmets. In 1991 the inventors company produced the Tempur-Pedic mattress made from this material. While used in medical settings it was discovered that the material helped with lessening bed sores for patients that had to lay in bed for long periods of time.
Memory foam has a cell structure that is open allowing air flow and along with the heat activated properties of the foam make it form around the body relieving any pressure points.
The fact that the foam is made of polyurethane means it gives off an odor which decreases over time.
I found www.bigdogbeds.com to be a great resource on the internet for information about memory foam dog beds. They offer human grade memory foam made for the medical professions with lifetime guarantees. Size and thickness based on the weight of the dog are the metrics used to pick one of these out. They are guaranteed not to sag. Really about twice the price I would pay for a foam bed but I really think that they are worth the extra money for the comfort of my dogs.