Pig slaves were genetically altered-mutant human-pig hybrids. They were both the slaves and creations of the Cult of Skaro, following the Cult's emergency temporal shift to New York City in the early 20th century. As servants, pig slaves were typically used for intense labour and strong-arm work.
Pig slaves only lived for a few weeks and were humans before the Cult converted them into beasts to abduct more humans which were then used to capture more humans and so forth, had little bits of white porcine-like hair and couldn't talk, only squeal and grunt.
When the pig slaves captured humans, the Daleks performed an intelligence scan on each of them. Humans of high intelligence were used for the "Final Experiment", while those of low intelligence were turned into the pig slaves. One individual, Laszlo, was only partially transformed. (TV: Daleks in Manhattan)
The slaves only lived for a few weeks (save Laszlo, as he was not completely transformed). They were several times stronger than the average adult human, being able to restrain a person with ease. They were also capable of biting through human flesh with their very sharp teeth — even slitting someone's throat with them proved easy (at least according to Laszlo's claim about them). They were killed with firearms when they charged at Hooverville and more were killed by Martha Jones, Frank, Tallulah, and Laszlo by using conductors positioned rightly for when the gamma strike struck. (TV: Evolution of the Daleks)