Let's go to the horses mouth (http://marvel.com/universe/Thanos) for the definitive skinny. Taking a paragraph at random:
Later, Thanos was trapped in a pocket of reality. Parnival Plunder, brother of Ka-Zar, stumbled onto Thanos' reality after contacting the being Consumption, and Thanos set up Parnival to be his pawn. Under Thanos' advice, Parnival created machines that would destroy the Savage Land, but in fact, Thanos absorbed the energy that was released by the machines so that he might open more portals and escape to normal reality. Ka-Zar discovered the truth and battled Thanos directly, and Thanos was toppled into a volcano through one of the portals. He managed to remain stuck in his pocket dimension, however, although he tried to get out by using first the Hulk and then X-Man as mental vessels. The two fought each other due to Thanos' influence, and although the villain managed to return to Earth after such a climactic battle, the two turned on Thanos and forced him to return to the pocket reality once more.Quite so. A story crying out for Ken Loach if ever I've heard one.