Normally miniature copies of the magus that he uses to perform tasks (flexible in accordance with the thought patterns of the magus) while busy working on his research. Acts as the magus would as if it were a part of him, although it is an independent entity (which is generally the best type of familiar). The symbol of the magus itself and fundamentally different from creatures binded by summoning and contracts. Physical and mental connections exist between the magus and the familiar.

Takes too much work to make a human being familiar from scratch, as well as to maintain it, which is why small animals are preferred, like dogs, bats, or cats.

Basically an aspect of the magus with the same knowledge but different personality and form of reasoning to serve as another way of gaining a different perspective on things.

Can be prepared by placing the lingering remains of a person's soul into the body of a dead animal. The soul is used to allow the smooth formation of a personality. This isn't resurrection, though. The resulting creature is something totally different. Following that, the corpse is repaired and a connection is added to the corpse to make it an extension of oneself, which is accomplished by moving parts of the practitioner's flesh (the parts where circuits are particularly strong in the caster; eyes, hair, blood, etc) onto the animal.

The resulting familiar will gain circuits that the original animal had never had before and can use spells just like the magus who created it. However, Prana cannot be created independently. This is supplied by its creator through their spiritual line. Hence, the magus is needed for the familiar to cast spells as well as survive. Some of the abnormally powerful familiars (those like Len who have a demonic aspect) can overcome this and form contracts with others to survive if their creator dies. These can also take Prana through other methods if they have no master.

That of course is only one type of familiar and many magi really don't like that method. Actual living animals can be binded to be made into familiars instead as well. There are also those that would make a familiar out of another living human being, but that's seen as monstrous. Also, familiars without souls can be used; however, the modern versions of these are weak.

In terms of relationship, the ideal familiar is not supposed to act without the orders of the magus.

Transference of consciousness into familiars is possible. Hence, the magus can use them like remote control units for recon or exploring. However, this isn't possible with familiars that have souls as they are the ones that are controlling the body. Just obtaining a video-feed from the familiar is also possible (like Kotomine did with Lancer in HF).

While it is possible to transfer one's consciousness/soul into a creature with an existing soul, this is nearly impossible. Even transference into a familiar without a soul doesn't mean that the magus has completely transferred himself. Destruction of the body (this also includes through natural means like aging) of the magus will mean the destruction of the soul of the magus. There are ways to get around this, for instance, complete transference using the Dress of Heaven, resurrection (Roa), or vampiric methods that Zouken had resorted to (and even then, it's not clear whether Zouken transformed or transferred his main body into a collection of worms).

Nrvnqsr's 666 beasts (or rather body itself) are also familiars. What makes these particularly different from other familiars are that they all are generally equal in power (each is Nrvnqsr himself would be one reading of it in relation to the above).