The Fundamental Particles

The atom is what we generally think as the smallest unit of matter. But now, it is known that it isn't, but it is the smallest piece of a certain matter that is recognizable as a unit of that matter, which we call an element, i.e. you can distinguish a Hydrogen atom from an Iodine atom by looking at its weight and its contents. This is what determines its position in the Periodic Table.

An atom's contents however is what is interesting for particle and nuclear physics. Upon dividing it further, one finds that it contains electrons, neutrons and protons, all of which are quite well known. However, the proton and the neutron are not non-dividable as might have been thought for a while in history: it is discovered that they consist of even smaller particles.

Also, it's important to know that there are composite and elementary particles. Elementary particles are the ones which current physics deems non-dividable and composite particles are composed of elementary particles. The neutrino and the electron are examples of elementary particles, both lepton type, and the neutron and the proton are examples of composite particles, both baryon type. The name baryon indicates that they are composed of 3 quarks. Because they are composed of quarks they are also called hadron. Hadron is an umbrella term for all particles that consist of quarks (also elementary particles) held together by the strong force. Mesons, which are also hadron, are made of 1 quark and one anti-quark. A pion is an example of a meson.

There are a whole lot more of these and classification depends on the type of spin (half integer, integer, top, down), to which fundamental forces/ interactions they are subject (electromagnetism, gravitation, strong interaction, and weak interaction), whether they are subject to the Pauli-exclusion principle (fermions) or Einstein-Bose statistics (bosons) and quite possible more that I am at this moment unaware of: I have yet to take quantum mechanics and statistical physics, therefore if I have made a mistake, then please let me know. You can find them in the standard model of particle physics.

Fermions: there are two types, quarks and leptons. There are 6 different types of quark and 6 types of leptons, with their corresponding anti-particles, making a total of 24.
Bosons: Photons, Z and W bosons, Higgs boson, Gluons.