A familiar is another word for a spirit companion, though there are slight differences. There are many tales of familiars in the days of old. It is quite common amongst those who walk the magical paths to know of their familiars and to know who their guides are. Spirit companions are not a new thing, they have been with us since the beginning and they will be with us till the end. The witches familiars, the animal guides who walk with and serve the witch. That is where the difference lies.
A Companion is a spirit being who has their own essence and presence. They have their own lives and that life can be apart from yours. They usually exist in another plane of reality and another layer of the spiritual realm and they usually have their own lives. Though they are a part of your life and will assist you in your life they are not there for the sole purpose of serving you. They are more then happy to help you and they will and many companions do come through and work with us for our lives here, but they still are vast and complex beings with their own lives. Companions also usually are outside of the realm of our spiritual world and so have a greater power of influence over the realms, and shifting them.
Familiars on the other hand are tied to this realm, or have limits of the realms they can move through. They exist within these planes and dimensions and they are here to walk with you and serve you. It is their purpose and their mission. They walk side by side with you and will act as your guides when you need them and perform the tasks that are laid out for them. Familiars exist within the spiritual plane that is connected to the realm that we reside in. They have come forward for the sole purpose to act as guides and aid the one they are with. They will work with you, know you and guide you on this journey. They can offer wisdom and council, and help you to shift the world around you.
We also have a mini booklet written by Priestess Akelta which answers this in more detail... It can be downloaded here...