OpenCL is an effort to make a cross-platform library capable of programming code suitable for, among other things, GPUs. It allows one to write the code without knowing what GPU it will run on, thereby making it easier to use some of the GPU's power without targeting several types of GPU specifically. I suspect it's not as performant as native GPU code (or as native as the GPU manufacturers will allow) but the tradeoff can be worth it for some applications.
It's still in its relatively early stages (1.1 as of this answer), but has gained some traction in the industry - for instance it is natively supported on OS X 10.5 and above.
