A great question! And one that is often misunderstood. No, not all cats (or dogs or toads, etc.) are familiars. In many cases the belief that a beloved pet is a familiar is incorrect.
The word familiar means “close companion” and is used to describe a spirit, imp, entity or demon (daemon) that inhabits an animal’s body. This entity is generally called to service by the witch and doesn’t come “pre-installed” in the animal. A familiar serves or guards the person who cares for it. Like many forms of magic, the familiar is fed by the witch, usually in the form of love or energy. In the 16th century, it was believed that the familiar was given to the witch by the Devil himself. The familiar fed upon the witch by sucking blood or some other form of life force from her fingertips or a hidden nipple.
Familiars can be astral or physical companions and can be used to guard your family and home. They should be released when their work is done, or when the animal has died.