What are valves? Why buy valves?
Valves are used to let a mixture of air and fuel into a cylinder and allow exhaust gases to get out. Each cylinder has at least two valves each, multi-valve engines have more, often four per cylinder. Upgraded head valves are able to withstand more heat from nitrous oxide use and turbo/supercharger use, so it's an excellent upgrade for people looking for added compression without sacrificing any reliability.

How do you install valves?
Valves require headwork and should be left to the pros. While replacing valves, valve guides, retainers, valve springs and camshafts should all be considered for upgrades at the same time.

What are valves? Why buy valves?
According to Wiki, "In valve setups, there are intake valves and exhaust valves. On the intake side, the intake valve opens which lets the fuel/air mixture into the combusion chamber. At some point (measured in degrees of crank rotation) the intake valve closes, the air/fuel mixture is compressed and ignited by the spark plug. Then the spent mixture exits the cylinder when the exhaust valve opens. Valve configuration varies somewhat, but regardless of how many valves there are there will always be at least one intake and one exhaust valve. Aftermarket valves allow for more air and fuel travel thru, thereby increasing horsepower.