GPU's are really good at parallel tasks such as calculating the color of every pixel on the screen, or doing the same operation on a large dataset. According to Newegg, the GTX980 has 2048 CUDA cores (parallel processing cores) that run at ~1266 MHz as opposed to a nice CPU which might have 4 cores that run at 4 GHZ. In other words, if you want to manipulate a whole bunch of things in one way in parallel, you can program it to use the GPU effectively, if you want to manipulate one thing a whole bunch of ways in series, CPU is your best bet.

(note: this is massively oversimplified)