A "no hub" connector is a fernco that is clad in metal. Also called a "Mission-Clay Coupler". It is used to connect 2 pipes together when only straight pipe is used.

A hub is also a bell that the the end of one pipe fits into on the other pipe and then sealed by a gasket or gasket material to make a water tight connection.

The no hub is only 1 step above a fernco which should only be used for short term or temporary repairs in emergency situations.