There are several ways to approach the basic question:
"What should we see on the display screen?"
The Classical Rendering Pipeline:
"How does each polygon appear on the screen?"
Ray-tracing:
"What do we see when we shoot a sampling ray through
each pixel?"
Radiosity:
"How do the photons from the light source(s) travel around
the scene?"
Volume Rendering:
"How do the photons interact with the medium through
which they travel?"