My husband once had a Familiar, a calico cat named Halloween. She was like our baby. We recently had to give her a new home with a close friend of our because we're moving. If we had been moving in the state or even to one of the surrounding states we would have kept her but, sadly, I don't think she would have faired well in a month-long road trip. Plus, I've heard that I might not be able to take her over Canadian boarder and there's no way I was gonna dumper her out of the car in North Dakota.
So what is a Familiar?
I believe they are animals we meet in this life, on this plane, who help guide and protect us. They can be cats, birds, dogs, lizards, mice, etc. They don't even have to be pets.
I had a crow that used to stay outside my window all winter and summer when I lived in Kentucky. I dreamt he would talk to me, tell me things, and in the morning he flew above me as I got on the bus or in a friend's car to go to school. When I got back, the crow was still there. I think it was a guide that I needed at that time. By the time I reached senior year of high school, the crow was gone.
The Difference between Familiars and Totems or Spirit Animals?
I believe a Familiar is or once was an animal on this physical plane that connected with a person.
A Totem or Spirit animal doesn't ever have to have been on this plane nor does it have to be an animal that even exists on this plane (i.e. 'mythical' creatures like dragons and unicorns).
Both Familiars and Spirit Animals are still very important to a witch's growth.