The Exhaust Manifold is a cast, branched tubular alloy unit, which is attached to the cylinder head, the shape corresponds with the position of the exhaust valves in the head. The exhaust emissions from the engine pass through the manifold which connects to the exhaust pipe system.
On a V configured engine there will be two Cylinder heads and therefore two exhaust manifolds which are bolted to the side of the heads. As the exhaust valves open within the head the fumes are forced into the manifold and into the exhaust system.
The reason for changing the exhaust manifold is usually due to leakage through a hole or crack caused by heat and corrosion. A new sealing gasket is required for replacement of the manifold.