This is the intake manifold. It supplies air from the intake/air filter, into the engine. Once the air enters the intake manifold, it is then channeled into 4 separate runners, one for each cylinder, and then it enters the engine and into the combustion chambers.
Bolted to the left of the intake manifold is the throttle body. The throttle body opens and closes when you are on and off the throttle. There are several vacuum lines and/or hoses connected to the manifold as well. On top of the manifold is where the throttle cable is bolted before clipping onto the throttle body.
Intake manifolds can be either upgraded with an aftermarket sheet metal intake manifold or the stock manifold can be ported to gain a little more horsepower.
The 1G intake manifolds are larger than a 2G intake manifold. But, the 2G intake manifold flows better than the 1G intake manifold. The 1G and 2G intake manifolds are different and will not bolt up to the opposite cylinder heads. A 1G intake manifold can only be used with a 1G head, and a 2G manifold must be used on a 2G cylinder head.