I've always thought of familiars as a spirit, usually animal, that presents itself to me and then is a guide, teacher, or companion. I've had dog and cat familiars, some of which have been mine ("ownership" wise) and some have not, but there is a lifestyle connection there, usually some form of co- or close-habitation. To me they are different from totems or spirit animal guides, visitors, or protectors (which I've had as many animals). There is a soul-connection and deeper understanding that many other animals won't have with me. It's similar to past life recognition. They are familiar to me, and me to them - there isn't a "getting to know you" process (because it is skipped right over by the deeper understanding) besides maybe learning the familiar's history, quirks, physical attributes. I find that it is a mutually beneficial relationship as the familiars also want to be seen and acknowledged by adept humans (as they are limited in their communicative abilities). They are also still animals, and if they are domestic, require love and whatever normal care for that animal would be.
I've never investigated thought-forms but before reading this thread would have defined a thought-form as sort of semi-autonomous remnants of a spell (intentional or inadvertent) that remains and begins creating its own effects on the world. Sort of an impish spirit, but without a soul and a very rudimentary consciousness (enough to navigate and perform its function, but no analytical skills or decision-making abilities). From what it seems, the consensus is more that a thought-form is an intentionally created mind-being that performs a function and is somehow dependent on the creator for maintenance. I see it sort of like a creating a dollhouse, and the inhabitants of that dollhouse are extant beings that have needs, sort of like human needs, and we mimic our own maintenance processes onto them. For those of a Gen Y or later persuasion, The Sims computer game may illustrate the process of maintenance.