the manifold is the part that actally bolts to the cylinder head. it's job is to evenly distribute air/fuel to the cylinders. it's typicaly made of cast aluminum, however in the last few years some auto manufacturers have moved to a plastic manifold. The plastic used for manifolds is denser and manufactured under much better controls than the air filter assembly.
here is what a typical manifold for a 4-cyl looks like. The part at the bottom of the picture attached to the 'plenum' is the throttle body (butterfly). It's the part that opens when you put your foot down on the gas pedal. The flange on the right is where it bolts to the engine, and those little round black grommets are the seals for the fuel injectors.
In contrast,
here is what a v-8 carburated Pontiac manifold looks like. This is a typical race/aftermarket example that's designed for upper RPM power.
-Fadi
1991 Mazda Miata to beat Corvettes at the track
1999 Dodge Ram to climb through rocks and brush
2004 Honda Civic Hybrid to make up for the other 2