During the age of carburetion, and its successor, throttle body injection, intake manifolds were composed of a single piece of molded metal. This didn't become an issue until multi-port fuel injection debuted.
Ideally, an engine would have a single fuel rail, centrally located, which could feed however many fuel injectors were present on the engine. This wasn't a problem on an inline engine, due to the arrangement of the components. However, on a v-type engine, this posed a problem.
A multi-port fuel injection system runs best when the injectors are located close to the cylinders. On a v-type engine, there is a large mass of metal known as the intake manifold in the way, and no way to move the injectors closer. They couldn't be placed on the outside edges of the engines because that would run fuel lines near the extremely hot exhaust manifolds.
The solution proved to be rather simple - Separate the intake manifold into two pieces. The lower piece would contain the runners, or air passageways. On top of that would sit the fuel rail, with the injectors pointing into the runners angled towards the intake valves. The second half of the intake manifold would sit on top of the runners, and function as a junction to split the air coming from the throttle body into the number of distinct runners present on the lower manifold.
The lower intake manifold is usually referred to by its full name, and occasionally by a shortened version, the 'lower'. The upper portion became named the plenum.
Modern plenums are typically manufactured with molded aluminum. Aluminum is extremely corrosion resistant, so unlike exhaust manifolds, they will not rust over. Mild corrosion will occur over time, but that won't effect the functioning of the plenum, and since it isn't a mechanical part, the corrosion is merely cosmetic in nature.
Bolts attach the plenum to the lower intake manifold, typically. Gaskets separate the two components. The throttle body then attaches to the plenum, and there is a gasket between these two parts as well. The gaskets exist to ensure the intake manifold as a whole is sealed. This allows accurate vacuum readings to be taken, if manifold absolute pressure is the method of air intake measurement. It also ensures that any air going into the engine has gone through the air filter.
Maintenance and Repair
Typically, the plenum serves as host to other devices, such as the EGR valve and PCV valve. This makes removal of the plenum for work on parts lower than it in the chain, such as the injectors, lower intake manifold, or cylinder heads, a very lengthy task which ends up costing a good deal of money.
Due to the EGR valve, the plenum will often become lined with carbon deposits. After 100,000 miles, it is a good idea to have it removed and thoroughly internally cleaned with something that can break up those deposits. Any auto parts store will carry various abrasive agents in spray-can form which will do the job nicely.
Simply squirt the inside thoroughly with the cleaning substance and rinse the remaining solvent away with a water hose, then allow it to dry. New gaskets for the throttle body, EGR valve, and plenum to lower manifold connections are a good idea, and an upper manifold gasket set can be purchase for most car models which will contain all of these gaskets.