It's worthwhile reading around before you ask questions. The wiki is a good place to start.


https://en.bitcoin.it/wiki/Why_a_GPU_mines_faster_than_a_CPU

There's even your exact question answered there.

Either way, ASICs mine 1000 times faster than GPUs as well, so both CPU and GPU mining are equally irrelevant in today's bitcoin mining.

See https://www.youtube.com/watch?v=GmOzih6I1zs