Building High Performance Headers | Getting the hot exhaust out of the cylinder is a primary process of a combustion engine.
If you can't get the exhaust out, you can't get fresh fuel and air in. In order to help the engine breathe, headers or exhaust manifolds are high on the upgrade list as these are the first connection to the exhaust ports of an engine.
The terms headers and exhaust manifold are often interchanged, but can be categorized differently. The stock exhaust on most street car engines is made from thick cast iron and is usually referred to as an "Exhaust Manifold". These manifolds collect multiple exhaust ports of an engine and merge the into and single outlet within a fairly short distance. The thermal properties of cast iron make it an ideal material for this purpose and it is durable.
"Headers" are usually made of tube steel and have much longer runners for the individual ports before merging together. Depending on the type of engine, the individual runners may be designed to be all the same length, or varying lengths.
The type of engine you have will greatly determine the type of exhaust you need. A naturally aspirated motor will be very sensitive to the design of the exhaust. Changing the length of the runners or the way they merge together can sway the power output by 30hp or more.