Compression fittings are built from three pieces, creating a mechanical joint connection fitting which is widely used in plumbing systems to join tubing together; a ferrule or compression ring, a compression nut and the fitting body (tee, coupler, elbow etc.)  The nut and ring are placed on the pipe, then the fitting is placed at the pipe's end. As the nut is tightened against the fitting the brass ring is deformed against the pipe itself, creating a strong seal.
Generally speaking, compression outlets are used for water and compressed air lines, where slight leakage isn't going to cause too much of a problem.  VMS stocks brass compression fittings in sizes ranging from 1/8” through 7/8” and in all available shapes including unions, changes of direction (bends, elbows), branches (tees), bushings, extensions, plugs, nuts, reducers, couplings etc., allowing for unlimited possibilities of connection.