The Yssgaroth's universe was an alternate dimension. The terrain appeared to be mostly desert. The universe had unusual proportions and distances, making travel even more difficult. There were many settlements of suffering humans, who were generally used as slave labour. It was the origin of Dream B.
Many species had been accidentally brought to the universe, including humans, Ogrons, Terileptils, the Galk and Bapputchin. Other species, such as the Cun, were either native or just unknown to the Seventh Doctor. Other species, such as the Hunters, could naturally travel to the Yssgaroth's universe.
The Time Lords originally discovered the Yssgaroth's universe during early experiments in making black holes. These experiments accidentally created ruptures in the space-time continuum, releasing the Yssgaroth. Other openings to the Yssgaroth's universe could be found in 1800s London, near Stonehenge and on the unnamed planet in the Althosian system. (PROSE: The Pit)