One part is vertex transformation which transforms 3D points into the 2D screen space, the other is computing the per-pixel colors. Both work on 4D vector data

One part is vertex transformation which transforms 3D points into the 2D screen space, the other is computing the per-pixel colors. Both work on 4D vector data though. Early GPUs had different processing units for vertex and pixel "shading", but over time those have merged into generic cores which can be dynamically assigned to vertex-, pixel- or 'compute'-tasks.