INTPs are perceiving types, and INTJs are judging types.
INTJ: Ni, Te, Fi, Se
INTP: Ti, Ne, Si, Fe
They have no functions in common, but they share an interest in abstract, logical reasoning despite being interested in and using very different aspects of it regularly. Ti tends to see everything in terms of a logical framework and nitpicks, Ni tends to have a "vision."
Another important thing to remember is that an INTJ is an INTUITIVE type first and foremost, while an INTP is a THINKING type first and foremost... meaning that their emphasis is different. Yet ironically, INTJs interact via Te and INTPs via Ne, so one would get the opposite impression at first.
I would conjecture that INTJs are more like strategists and practical achievers, and INTPs are more like engineers or philosophers.