A gasket is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression.  Gaskets allow "less-than-perfect" mating surfaces on machine parts where they can fill irregularities.  It is usually desirable that the gasket be made from a material that is to some degree yielding such that it is able to deform and tightly fills the space it is designed for, including any slight irregularities. 
Soft Gaskets
Semi-Metallic Gaskets
Metallic Gaskets