Glenn Mullin, translator of many Dalai Lama biographies and works of Je Tsongkhapa, responds to the question:
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.