INNERSPRING (The mattresses made popular by the "S brands": Simmons, Serta, Sealy)
What it is: Coil springs buried beneath foam, insulation, and cotton. Accounts for 90 percent of the industry's sales.
What you need to know: Lie down on it for ten minutes, then buy based on comfort. Don't pay more than $1,500. Price range: $200–$65,000

FOAM (Tempur-Pedic, Simmons ComforPedic)
What it is: Everything from standard latex to the NASA-developed visco elastic (memory foam) made popular by Tempur-Pedic.
What you need to know: Conforms to your body, relieves pressure, can allegedly store body heat and cook you. Price range: $800–$7,000

AIR (Select Comfort, Comfortaire)
What it is: Essentially the same as an innerspring bed, only with a balloon core instead of coils.
What you need to know: Reduces pressure points and late-night discomfort. Price range: $600–$5,000

SLEEPING GIANT (Sleeping-pad cover by Big Agnes)
What it is: A camp-worthy sleeping-pad cover made out of memory foam.
What you need to know: It's a Tempur-Pedic for the backcountry ($60; bigagnes.com).