What counts as a familiar?

So as at the title asks, what exactly counts as a familiar? Or are there rules as to what is a familiar for a witch? If so, how do you know you have one, or go about getting one/bonding with one?

Im new to witchcraft, I've just now reached a yesr since i started fhis journey and I've always known about familiars in movies and such, but im curious as to what one would actually look like?

Thank y'all in advance to anyone who answers!

(Also sorry if it isn't tagged right, I wasn't sure what to out this post under?)