A tulpa is what is called in Yiddish a golem.
It is a manifestation created by an adept to accomplish a certain task or tasks.
"Once the tulpa is endowed with enough vitality to be capable of playing the part of a real being, it tends to free itself from its maker's control . . . . Tibetan magicians also relate cases in which the tulpa is sent to fulfill a mission, but does not come back and pursues its peregrinations as a half-conscious, dangerously mischievous puppet. The same thing, it is said, may happen when the maker of the tulpa dies before having dissolved it."
~ Alexandra David-Neel, With Mystics and Magicians in Tibet.
"Nowadays, if you speak the truth, there is nobody to listen; if you speak lies everyone thinks it is true. I have never said this before: I am not an ordinary person; I am a bodhisattva who has taken rebirth through aspiration. The suffering experienced in this body is just the residue of karma; but from now on I will never again have to experience karmic obscuration. … Now, in this final age, the barbarians beyond the frontier are close to undermining the teaching. [So] there is no point whatsoever in my taking rebirth here…I have no reason to take birth in impure realms ever again.”
This may be interpreted as a statement that his mindstream would have no further 'emanations' (Wylie: sprul pa (tulpa); sprul sku (tulku)).
Sprul pa, basically is a Tulku, an nirmitaka, a relative emanation.
Often these terms become mixed with other meanings that lend to a "sinister" interpretation and continued lack of knowing. Same type of outside looking in, that call Tibetan tradition "Lamaism"
Really has nothing to do with our practice.
Both technically and in common useage there are numerous differences.
For example, technically tulku is one of the three types of nirmanakaya embodiments of a buddha. In Tibetan social convention, it is a child who is officially recognized as the rebirth of a lama, and is entitled to the inheritance from the predecessor.
In this latter sense, the tulku is just an honorific title meaning something like "lama by birth," with no requirement that the predecesor be especially enlightened. Many simple gesheys, abbots, etc can be granted permission to create a "reincarnate lineage," altho the process usually happens in reverse: the old guy dies, his students approach the head lama in the school and ask if it is useful to look for and identify a child as the reincarnation, and then the child who is recognized becomes called a tulku.
In contrast, a Tulpa does not need to be any of the three kinds of nirmanakaya listed by Asanga in the Abhisamayalamkara. He can be far more mundane emanations, like a dog, book or bridge; or more exalted, like a samboghakaya emanation.
Perhaps the relationship is what is called mu sum in Tibetan dialectics: all tulkus are tulpas, but not all tulpas and tulkus.