Texture mapping
Pasting an image onto a 3D surface — how a model of a few hundred triangles can look like brick, wood or skin.
Each vertex in the model gets UV coordinates pointing at a spot in the texture image. During rasterisation the coordinates are interpolated across the surface and the graphics card looks up the corresponding colour. Edwin Catmull described the technique in 1974, long before hardware could do it in real time.
Modern materials use several textures per surface: one for colour, a normal map faking small bumps by tilting the lighting calculation, and maps for how glossy or metallic the surface is. Unfolding a 3D model into a flat image is called UV unwrapping and is among the more thankless tasks in 3D work.