The skin was the outer covering of animals. According to the Eleventh Doctor, human skin was fragile, like parchment. (TV: Journey to the Centre of the TARDIS)
The Foamasi (TV: The Leisure Hive, PROSE: Placebo Effect) and the Slitheen (TV: Boom Town) used skin suits to disguise themselves as humans.
By the time the Ninth Doctor and Rose Tyler met Lady Cassandra O'Brien.Δ17, she was little more than a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain. After that piece of skin ripped apart, (TV: The End of the World) her brain was attached to stretched out skin taken from the back. When she was finally able to leave this body and enter Rose Tyler's, Cassandra said, "Goodbye trampoline, hello blondie." (TV: New Earth)
When Toby Zed was possessed by the Beast, his eyes were red and he had markings on his skin. (TV: The Impossible Planet / The Satan Pit)
Jenny was created in a progenation machine from a skin sample taken from the Tenth Doctor. (TV: The Doctor's Daughter)
One way that individuals reminded themselves they had encountered the Silence was to make tally marks on their own skin. (TV: Day of the Moon, The Wedding of River Song)
When Mels regenerated into River Song, her skin colour changed. (TV: Let's Kill Hitler) The General's skin colour also changed in at least one regeneration as well as Karlax. (TV: Hell Bent, PROSE: Engines of War)
Sleep dust largely consisted of skin cells. (TV: Sleep No More)