An Intake Manifold is the part of an engine that supplies the fuel/air mixture to the cylinders. In contrast, an exhaust manifold collects the exhaust gases from multiple cylinders into a smaller number of pipes – often down to one pipe. The primary function of the intake manifold is to evenly distribute the combustion mixture (or just air in a direct injection engine) to each intake port in the cylinder head(s). Even distribution is important to optimize the efficiency and performance of the engine.
It may also serve as a mount for the throttle body, fuel injectors and other components of the engine. The intake manifold gaskets are responsible for sealing the intake manifold against the cylinder head. Symptoms of a bad intake manifold includes engine overheating, engine backfires, coolant leaking etc.