An Interzone portal was a doorway between the zones on the planet Bortresoye, in the Divergent Universe. The Eighth Doctor, Charley Pollard and C'rizz used them to travel around that planet, while the TARDIS was temporarily lost to them.
The Interzone between the portals was a hazy white space with a very short sight distance, and a humming/buzzing background. Most denizens outside of the Interzone had no awareness of the portals' existence.
The portals were controlled by the Kro'Ka, who directed the travels of the Doctor and his companions at the direction of the Divergence, and later Rassilon. (AUDIO: The Creed of the Kromon, The Last, Caerdroia)