Physically-based rendering means that we're essentially applying the law of physics to the problem of representing materials and bouncing light on them. The ren

Physically-based rendering means that we're essentially applying the law of physics to the problem of representing materials and bouncing light on them. The renderer deals with well defined physical quantities such as meters, watts, etc. To some extent, the renderer produces images that match real world experiments or predict what a setup would look like if it actually existed (predictive rendering). Non-physically-based rendering ("classic rendering"?) is using ad hoc tricks to produce convincing images, but without following a formal framework. While recent AAA games all more or less follow the physically-based paradigm, older 3D games didn't and simply used custom models and tricks to produce good looking imagery.