Cats are the most common form of witch familiar, however various other animal forms have been used (Dogs, camels, guinea pigs etc.).
While in this form, a witch familiar has no access to any of the magic they had as a witch, but regain their magic once the sentence is served and human form is regained. However, the magic can be rusty and take the witch a while to regain control over, as seen with Duke.
Witch Familiars are made to live with a witch for the duration of their sentence. Occasionally the witch is selected due to their connection to the crime, for example: Hilda Spellman was made to house Salem Saberhagen because she was the head of the refreshments committee in his regime.