The intake manifold is a part of the engine intake system which is usually what a throttle body is bolted to. It is almost always made of metal, since it has to withstand the vacuum pressure generated by the suction of the engine. However, advanced plastics are becoming a more common intake manifold material.
Two Piece Intake Manifolds
Sometimes the desired intake passageways that the engineer/manufacturer comes up with is too complicated to cast as one piece. In other cases, getting access to some fasteners is improved by having two pieces, or even requires two pieces. In these scenarios, you usually end up with an upper intake manifold and lower intake manifold. The lower will be bolted to the cylinder head(s) while the upper will be bolted to the lower, and the throttle body will (typically) be bolted to the upper.
Aluminium Intake Manifolds
An aluminium metal intake manifold is usually cast using sand to shape the internal passageways, so those passageways tend to be rough when stock. Sometimes these are ported and polished to improve power, but this is typically done by performance shops and vehicle owners looking to get more horsepower out of their engine.
During the casting process, vacuum line connection points may be embedded in the mold so they are more durable; aluminium would be too thin/small for many vacuum lines, leaving them prone to being broken.
Plastic Intake Manifolds
While plastic has been around for a long time now, its widespread use in oter engine parts is a relatively new development. Typically, plastic intake manifolds aren't purely plastic; glass fiber reinforcement is common, as well as brass or steel sleeves where bolts will be clamping down on the manifold to keep it in place. The inner section where the intake air flows through tends to be smooth, which is (typically) an air flow benefit that would require extra machining on an aluminum intake manifold.
Intake Manifold Gaskets
The intake manifold typically attaches to the cylinder head(s), and keeping these surfaces sealed is important to prevent vacuum leaks. The gasket between the intake manifold and the head is referred to as an intake manifold gasket.
Upper & Lower Intake Manifold Gaskets
When there are upper and lower intake manifolds, an additional sealing surface is created, requiring at least one additional gasket. What would have simply been an intake manifold gasket is now a lower intake manifold gasket, and between the two manifold sections will be the upper intake manifold gasket. This additional sealing surface is also protecting against vacuum leaks, and sometimes just the upper gasket may be purchased if a service only requires the removal of the upper intake manifold.