What actually are the difference between a servitor and a familiar? I think that a familiar can be, but not necessary a servitor.
Regarding the differences between a familiar and a servitor, the primary difference is this:
A familiar is a real spirit either conjured by the magician from the dead or granted to the magician by the spirit's "boss" in the hierarchy. They may function as a servant, and that's fine.
Servitors, on the other hand, are usually just figments of a Chaos Magician's wishful thinking.
Servitors are a construct of the Chaos Magick movement. Chaos magicians generally believe in a blend of the energy and psychological models, that everything is made up of energy, and whatever we experience spiritually is the result of our own perceptions and beliefs, and that if it affects reality, there's probably a rational, scientific explanation that has no need for actual objective spirits to exist. They believe energy is manipulated by belief, and that a servitor can be created entirely from the magician's own expectations, energy manipulations, and the power of their belief.
How they can hold this philosophy is completely beyond me. If belief were the sole source of experiential reality, insane people really would be whatever they thought they were. I've experienced enough mentally ill people to know that no matter how true their faith in their delusions might be, their delusions are not real. Not one of the Napolean Bonapartes in Belleview lived in Elba. The homeless Viet-nam vet on crutches muttering about how he's the son of David, the last Sun King, really believes what he's saying, but that belief doesn't make him the brother of Solomon, nor does it transport him mystically and magickally to the streets of Jerusalem. Even though he REALLY believes that's where he lives. (This is a real guy, he lived in Denver when I was a teenager, and tried to convince me I was also a Sun King at a coffee shop one day after some punks had beaten him up and taken his vodka and spare change.)
That doesn't mean I don't believe some servitors are actual spirits. Take Fotamecus, for instance. This time-manipulation servitor allegedly became an egregore after being exposed to the energies of a rock concert (or something like that). As time went by, Fotamecus grew in power and was gunning for Chronos. Magicians across the world experienced Fotamecus.
However, the primary "creator" of Fotamecus has recently begun to understand that the spirit existed long before he was "inspired" to create the Fotamecus sigil and go through the operations he's experienced.
Similarly, studies of the different grimoires indicate that there are nephesh, shades of the dead that can be conjured and used as servants. When a Chaos magician creates a servitor and has results with it, I believe it's because some wandering shade has inhabited the form of the servitor and is causing the effects. Again, it's a real spirit; it's just taking advantage of the thought-form of the magician. This is evidenced by the experiences people who have useful servitors and tulpas have recorded.
Tulpas are familiar spirits from the Tibetan magickal systems that are allegedly created by the magician. The magician imagines the form of their spirit, and over time practices extensive visualization of the thing, empowering it, treating ti as if it were real, making offerings, and so forth. After a couple of months of consistent effort, the tulpa is as real as any familiar.
However, after a while, the Tulpa inevitably begins to change. It changes its form and function. The spirit that has inhabited the visualized form of the magician takes over the construct, and it begins to look and act the way its nature demands. It's not long before the spirit is obviously not what the magician imagined it to be, and the magician is then forced to eradicate the tulpa, a process that can take six months.
Now I know there are lots of people who think they have created tulpas, and who theorize about them based on the writings of charlatans and frauds, and they write a bunch of untested bullshit that doesn't work and publish it to the web. Google will provide hundreds of sources that will disagree with me, I promise.
Check into the experiences of people like Alexandra David-Neel, who spent some fourteen years in Tibet and actually created a tulpa. Compare their experiences and records with the theories and claims of the popular servitor/tulpa movement, and you'll quickly recognize the difference in tone. Truth strikes a chord that BS just can't. She and others who actually perform the ritual creation of a tulpa record the same end: the spirit changes, revealing its true (and usually disturbing, vampiric) form and the magician is forced to banish it from their lives.
Before anyone gets too upset, I'll close with the caveat that your mileage may vary.