Compression fittings are used in Plumbing to join two pipes together. Because in plumbing pipes are made of different materials (most commonly PVC and copper, so too are the fittings.

Compression fittings work by the application of force to the pipes, in other words by pressing the fitting so hard against the pipe that no space is available for water to leak through.

References

http://www.diydata.com/techniques/plumbing/joints/compression/compression.htm