This is a bit of an over simplification, because the 'cores' of a GPU aren't really comparable to a CPU core. It's more like scaling up a factory - at a single point on the production like you go from 1 person doing a job to 100 people doing the same job with the same tools. It means you can do more of the same thing, but can't do 100 completely different things.
Here is a good overview:
http://gamedev.stackexchange.com/a/17255