Material

A material is the recipe for how a surface responds to light: base color, roughness, metalness, transparency, emission, plus whichever textures feed those slots. The mesh says 'this is a teapot shape'. The material says 'and it is brushed copper'. Same geometry, different material, and viewers read it as a different object.

Most web 3D uses a PBR material (physically based rendering), where the two sliders that do the heavy lifting are roughness and metalness. Metalness is close to binary in practice: something is metal or it is not, and half-metal values mostly produce muddy plastic. The cheap alternatives are worth knowing too: unlit/basic materials ignore lighting entirely (great for UI-ish 3D), matcaps fake a whole lighting setup with a single sphere image, and toon materials quantize the shading into flat bands.

Gotcha: a material can only look as good as the light hitting it. People tweak roughness for an hour when the real fix is an environment map, because reflective surfaces have nothing to reflect in an empty scene. Second trap: each distinct material means its own draw call, so eleven slightly different metals on one model cost more than one shared metal with a texture driving the variation.