the classification of materials
There are millions of materials, but they fall into a handful of big families that share a family resemblance in bonding, structure, and behavior. Sorting a new material into its family gives you a fast first guess about how it will act - much as knowing an animal is a bird tells you it probably has feathers and can likely fly.
The traditional classes are metals (iron, aluminum, copper - strong, stiff, ductile, conduct heat and electricity, shiny), ceramics (compounds of metals with oxygen, nitrogen, or carbon - hard, stiff, heat- and corrosion-resistant, but brittle), and polymers (long carbon-based molecular chains - light, flexible, cheap, easy to shape, but soft and low-melting). Two combination or special classes round it out: composites (two of the above combined, like carbon-fiber-reinforced plastic) and semiconductors (silicon and its kin, with electrical behavior between conductor and insulator, the basis of all electronics). Glasses are usually grouped with ceramics but are non-crystalline.
The class explains the property. Metals are ductile because their bonding lets atoms slide (dislocations move); ceramics are brittle because their bonds resist that sliding; polymers are floppy because only weak forces hold their chains together. Modern fields - advanced, smart, and biomaterials - cut across these classes for special functions. Classification is the first sorting step in any materials selection.
A coffee mug shows three classes at once: a ceramic body (hard, heat-resistant, brittle - it chips if dropped), a polymer handle grip (soft, warm to the touch), and maybe a metal spoon (bends without breaking). Each behaves true to its family.
Three material classes, three behaviors, at one breakfast table.
The families overlap at the edges - glass-ceramics, conducting polymers, and metallic glasses all blur the lines. Classification is a useful first sort, not a rigid law of nature.