http://www.apsense.com/article/what-are-the-purposes-of-the-cylinder-head-cylinder-head-gasket-in-the-engine.html
The Cylinder head gasket is one of the most critical parts of the engine. It is mounted between the cylinder head and engine block and its primary function is to maintain a seal between the two parts. The head gasket basically contains coolant in the relevant passageways, prevents air & combustion gas leakage, and maintains compression of the engine cylinders.