What is the difference between these two graphics cards?
A 780 has fewer texture units and cuda cores than the Titan, but not by much. A Titan has 2688 Cuda cores, while a 780 has 2304, the 780 also has 3GB Vram vs 6GB in the Titan. There is roughly a 10% performance difference between the two, though a 780 can easily be overclocked to almost match a stock GTX Titan in terms of performance. There's also a $350 price difference.
While its true that as far as games go there a 780 can be pushed to be near the Titan, some people are missing the point of the Titan. The Titan has the same GPU as the Tesla compute cards, and if you are doing things on your computer that utilize CUDA, the Titan delivers far, far more bang for your buck.