In very simple terms, a pixel pipeline will normally have a single pixel shader on it working on a single pixel. (some cards like the x1900 series had 3 engines per pipeline.) It can't do anything other then shade pixels. Older cards needed separate pixel and vertex shaders. Cuda cores don't care if its a pixel or a vertex, they can do either. It's also possible for them to perform other calculations like Nvidia has for PhysX. Cuda cores can perform the same function as the old pixel pipelines but can do a great many other things.