What is a familiar?
Familiars are most commonly cats (and they tend to make the best ones), though some very powerful historical sorcerers have used other creatures and animals. Familiars choose their owner rather than the other way around and a bond is created between sorcerer and familiar. Familiars who have chosen a sorcerer cannot be used as a conduit by anyone else and a sorcerer cannot just choose an animal to act as a conduit; without the bond, the attempt could be deadly to either animal or sorcerer or both.
How does this bond work?
The bond between animal and sorcerer runs deep and lasts for life. There are many cases where the death of a sorcerer has caused the death of a familiar, because for some the bond is so strong. In most cases the familiar has an innate urge to protect the sorcerer in any way possible and more often than not the familiar will pass before the sorcerer, though the death is often just as painful. Professor Klein, a specialist in familiar magic, has described the loss of a familiar as "akin to losing one's limb". The bond, though, does extend the life of the familiar and can extend the power of the sorcerer.
What are familiars used for?
Some spells require a lot of energy and a lot of stamina, using a familiar as conduit for a spell can help relieve some of the pressure that is on the sorcerer by using some of the energy of the animal rather than overexerting themselves.