General Purpose GPU
The General-purpose computing on graphics processing units, or GPGPU is the technical name for your GPU when it's used to do OpenCL or CUDA calculations. Parts of the hardware inside the GPU can be used for rendering graphics, but also to do intricate calculations that would normally be done on the CPU. The speed increase of using the GPGPU comes because it's got several tens to thousands of rendering cores that are simultaneously used to do the calculations with.
A task from a project is split up in little chunks that all the processors in the GPU calculate in parallel.