The Slitheen first appeared in the two parter “Aliens of London”/”World War Three” where the Slitheen Spaceship crashed into Big Ben and the Slitheen invaded 10 Downing Street. Margaret Blaine (A Slitheen) then reappeared in “Boom Town”, 6 episodes later where Margaret Blaine becomes The Mayor of Cardiff. The Slitheen Spaceship can be seen below.
The Slitheen are inhabitants of the planet Raxacoricofallapatorius. Slitheen is the family name not the name of the species. Made up of Calcium, the Slitheen have a strong weakness to Acetic acid, an example of which is vinegar.
Compression fields are around the Slitheen’s necks so they can literally fit into skins of humans as a disguise. This is much like the Tardis, both the Tardis and the Slitheen when in human form are bigger on the inside. Due to being compressed in a human skin, the Slitheen lack calcium and suffer from Calcium Decay which The Doctor and Rose pointed out that there exchange of gas smells like bad breath. Below you can see a Slitheen unmasking.
In “Love and Monsters”, Series 2 Episode 10, The Abzorbaloff explained that there is a twin planet to Raxacoricofallapatorius. The twin planet of Raxacoricofallapatorius is Clom, the home of the Abzorbaloff. The Slitheen and Abzorbaloff both look quite similar as well.
One Slitheen had taken the disguise of Margaret Blaine and eventually is The Mayor of Cardiff. At the end of “Boom Town”, Margaret Blaine looks into the heart of the Tardis, turning her into an egg (can be seen below).
The event causing the Tardis to open was the fact that Margaret Blaine was standing on the extrapolator ready to surf through space, opening the rift as she described it, this destroying the Tardis however the Tardis is a living thing and opened. This is when Margaret Blaine stared into the heart of the Tardis, giving her what she wanted hence forth her turning into an egg. The Slitheen also had a small appearance in a special interactive mini episode of Doctor Who called “Attack of the Graske”.
Margaret Blaine still lives and was took back to the planet Raxacoricofallapatorius to have a new life, a second chance. The other members of the Slitheen family were killed. The Raxacoricofallapatorians may come back though or maybe even Margaret Blaine.