There are three types of familiars.
Astral, physical, and artificial.
The astral represents more of the "totem animal" concept. This familiar will greet you through meditation and workings. This familiar also can reveal itself in a certain animal form through such states of mind. This familiar carries out tasks that give it through other planes.(ritual workings, and the such.)
The artificial familiar is one that you create yourself, to do the similar tasks.
The physical could be things as a pet, but you cant send it through different planes to carry out workings.
What most people don't understand is that when working with the familiar, its a "give and take" relationship. Its not a "master and servant" relationship. The astral needs you to do things as well, and they could even become a bit rebellious. The artificial needs energy, and to be charged in order for it to carry out tasks. And as for the physical, you still have to feed the poor animal, give it affection as it does you.
Hope this helps.