A tulpa, or a thoughtform as I prefer to call it, can be formed if you believe strongly that something exists. This is because the energy that you radiate when you think about it, believe in it and say "this thing with this and that trait is real" is directed in such way, that is slowly concentrated in one "place" until it is molded and shaped in the way that you will it.
In the end, this blob of energy in the end acquires consciousness.
In a nutshell, a thoughtform is an artificial spiritual being. Think of it as building a human with total control over his appearance, personality, skills and flaws etc... The fact that it is artificial however doesn't make it lesser or higher than other souls, as a thoughtform too can be humane, so treat yours with love and respect.
A spirit on the other hand, is not created by another entity, at least not with the same way as one would create a tulpa. It can be an energy expression of nature (e.g. elemental spirits), the universe, an non-directed accumulation of a certain kind of energy/energies, a byproduct of two or more souls or spiritual beings fusing, clashing or shattering, etc.
I hope my words helped you understand, and didn't make you even more confused.