Isolators and Circulators
A circulator is a 3-port device that transfers electromagnetic energy between the ports A, B, and C only in one direction: A to B, but not A to C. B to C, but not B to A, C to A and not C to B. This circular connectivity of the ports can be reversed with switching the polarity of the magnetic field. The circuit is often made by joining the three transmission lines with 120 degree angles netween them, thus this design is often referred to as Y-junction design.
Every Circulator can be made to work as an isolator by terminating one of the three ports with load. The remaining two ports are now "isolated": Energy in transmitted from A to B, but not from B to A. The isolation can never be better than the inout match of the load used for termination.
The Faraday isolators use the Faraday effect of rotating polarization planes in gyromagnetic media. These two-port devices only work as isolators, but they can be made switchable.