elementary vs composite
Hold a sand dollar and a marble. The marble looks solid and simple; the sand dollar, if you break it open, spills out smaller grains. Some objects are single, indivisible things, and some are bundles of smaller things stuck together. Telling these two cases apart is one of the central jobs of particle physics, and the words for the difference are elementary (no parts) and composite (made of parts).
A particle is elementary if every experiment shows it as a structureless point, and composite if it is a bound state of smaller particles held together by a force. The decisive test is to probe the particle with something very small — typically a high-energy beam acting like a tiny ruler. If the probe passes through smoothly, the particle looks elementary; if the probe sometimes bounces off hard, sharp lumps inside, the particle is composite. This is exactly how the proton, once thought elementary, was revealed in the 1960s to contain three smaller quarks: high-energy electrons fired at protons scattered off hard points inside.
The distinction matters because it tells us where the real foundations of matter lie. Atoms are composite (nucleus plus electrons); nuclei are composite (protons and neutrons); protons and neutrons are composite (quarks and gluons); but quarks and electrons appear to be elementary, the bottom of the ladder. An honest caveat: a composite particle is not just a loose bag of parts. Most of a proton's mass comes from the energy of the strong force binding its quarks, not from the quarks' own masses — so a composite object can weigh far more than the sum of its pieces.
Fire low-energy electrons at a proton and it acts like one smooth blob. Fire very high-energy electrons and some bounce back sharply, as if they hit tiny hard nuggets inside — the quarks. That change in behaviour is the experimental signature of a composite particle.
Hard scattering at high energy reveals the hidden parts inside a composite particle.
A common misconception is that a composite particle weighs the sum of its parts. It does not: for the proton, most of the mass is binding energy from the strong force, not the rest masses of the quarks inside.