It uses the same GPU as the Titan XP, with the same number of cores. However it has a higher clock, resulting in better performance. It has a slightly narrower memory interface (and therefore 11 GB of VRAM vs 12 for the Titan), but again a higher memory clock, resulting in roughly the same memory bandwidth.
Overall the 1080 Ti is a little more powerful than the Titan, but the difference in cost is the part that's really significant.