Hell was believed by some cultures to be the place of punishment after death for those who had sinned in life.
Hell was embedded in the minds of species like humans through religions like Christianity, the opposite afterlife being Heaven. The Seventh Doctor claimed there was no such place as Hell, but he probably spoke in terms of the popular concept of Hell. (AUDIO: Gods and Monsters)
Hell was often used to describe greatly uncomfortable situations. Morbius described his deteriorated state as hell, (TV: The Brain of Morbius) and the Tenth Doctor later described the Last Great Time War as hell. (TV: The End of Time)
Real Hell edit
While it was a concept, Hell was also a plane of existence or universe somewhere in reality that had a fiery geometry. The Scourge lived here and tortured the humans of their universe. (AUDIO: The Shadow of the Scourge)
Jason Kane spent several years in Hell, described as predominantly rock where tunnelling craft tunnelled their way between bubbles of places which weren't rock, which would in theory be the equivalent of planets. This may have been the Infernal Regions, however, which is probably where the idea of Hell came from. (PROSE: The Infernal Nexus)