The turbine housing is typically made of ductile cast iron or cast nickel iron. Its main function is to direct the exhaust gas from the engine onto the turbine wheel, and to contain the gas as it passes through the blade passages of the wheel. The gas passes through a volute (the snail-shaped section) and as a result of this the wheel rotates, and the shape and size of the turbine housing increases the speed of the gas as it meets the turbine wheel.
Turbine housing size is normally referred to by the term A/R, which refers to the volute throat area related the size of the turbine wheel. The A/R of the housing adjusts the flow capacity of the turbine, therefore, using bigger or smaller A/R will impact turbine performance. For example, the use of a larger A/R will decrease the exhaust gas speed which will allow more flow at higher engine speeds and more power overall, but reduce the boost pressure at low speeds. On the other hand, using a small A/R reduces the flow capacity which gives better response but leads to increase exhaust backpressure and impacts peak engine power.
The turbine housing is an essential part of a turbocharger because if it cracked or worn out the turbo will not generate boost pressure correctly.