These refer to the Arithmetic and Logical Units (ALU) of the GPU which act on the shading instructions. These are called Stream Processors. In earlier GPUs (prior to the GeForce 8XXX series and the ATI 2XXX series), stream processors existed separately as Vertex Shaders and Pixel Shaders. Stream processors are completely programmable and can be used for either of the shading jobs. These can operate at a different clock speed from the other components of the GPU.