Rasterisation is one specific step of the rendering process (+). GPUs rasterise triangles. If you're writing a software renderer you can include algorithms to directly rasterise other shapes such as circles, which is what started this whole thread. But if you're rendering with a GPU and relying on its built in rasterisation then triangles is where you start, even if circles is what you ultimately want.
(+) It's also possible to render without the rasterisation step at all - for example, a pure ray tracing based renderer doesn't have an explicit rasterisation step as it's normally thought of in graphics terms.
