To add to what Lark said, a familiar is merely a spirit friend or assistant in a way, and can be anything, not just an animal. In fact, most familiars are not animals. I do however find that cats are natural mediums and this may be why cats were commonly viewed as the witch's familiar. Kindred spirits are usually confused or used interchangeably with familiars.
Those that are created (thoughtforms/egrigore) are soul-less spirits.
When a witch draws energy from a spirit, if drained entirely, the spirit doesn't truly "die" but sleeps in a way until their energy naturally rebuilds or the witch heals them by replenishing the taken energy later. I know Lark knows this, but I can see where so many may take that the wrong way. Spirits can't die!!! No slaying spirits please, that's for RP.
Thoughtforms are commonly destroyed and recreated, and truly should be, or else they can become hard to control and tainted. Drawing from their energy and restoring it later would keep the created spirit the same but constantly renewed so that it would not go sour.