The Special Weapons Dalek appeared in Remembrance of the Daleks, a 1989 Doctor Who story during the Sylvester McCoy era.
Ordinary Daleks have an eyestalk and two 'arms' - usually a gun (which looks like an egg whisk) and a manipulator (which looks like a toilet plunger). The special weapons dalek, on the other hand, just had a bloody great cannon.
The Special Weapons Dalek was one of the Imperial Daleks, so it was coloured white with gold nodules. However, while its fellow Imperial Daleks were the picture of gleaming perfection, the Special Weapons Dalek was smeared with grease and dirt.
Ben Aaronovitch wrote both the script and the novelisation for Remembrance of the Daleks. In the book, he gives a little more detail about the Special Weapons Dalek. He describes it as a one-of-a-kind Dalek, created by Imperial decree. Driven insane by the radiation from its over-powered gun, it has developed a sense of self. This is blasphemy amongst the other Daleks, who call it the Abomination and fear it. Only the will of the Emperor Dalek keeps it alive.