The intake is a system of components that manages air going into the engine. It usually begins with the air filter, passes through tubing which houses air intake sensors (such as the mass air flow sensor), to the throttle body, then into the intake manifold, through a cylinder head, finally reaching an intake valve where, when the valve opens, the air will enter the combustion chamber. Unless the engine is a direct injection model, somewhere along the intake (usually after the throttle body) the fuel will be added to the mix.
Air Flow
The more restrictions there are in the intake system, the more difficult it is for the engine to draw in air, which reduces overall power. Sometimes a port and polish is performed to improve air flow. Porting opens up the air flow passageways, often removing some restrictions and casting marks. Polishing smoothes the remaining surfaces, altering the way air flows near the walls of the intake components.
Helmholtz Resonator
Found more commonly on modern vehicles, Helmholtz resonators are extra chambers found on intake tubing that allow engineers to tune the sound of an engine, often giving it a sound with more mass-appeal.