Bohr developed an improved model of the atom to explain the absorption and emission line spectrum that could not be understood by the Rutherford atom model. The Bohr atom is similar to Rutherford atom, except the electrons can only move in fixed or quantized orbits. That is, they can't orbit just anywhere, like planets around a star.
The quantized orbits of the electrons allows for a simple explanation of the origin of photons, and the spectrum of light. Photons are produced by the transition of electrons downward in their orbits. A downward transition releases potential energy in the form of a light particle, a photon (as seen in an emission line). Likewise, photons could be absorbed by electrons (causing an absorption feature), and they move upward in their orbits.
The two properties of the Bohr atom led to the development of quantum physics and the detailed understanding of the emission and absorption nature of spectra.
Physics recognises four fundamental forces of nature:
In the 1930s, it seemed that protons, neutrons, and electrons were the smallest objects into which matter could be divided and so they were called "elementary particles". The word elementary then meant "having no smaller constituent parts", or "indivisible" -- the new "atoms", in the original sense.
Later physicists discovered yet another layer of structure within the protons and neutrons. It is now known that protons and neutrons are made up quarks. Over 100 other "elementary" particles were discovered between 1930 and the present time. These elementary particles are all made from quarks and/or antiquarks.
There are six quarks, but physicists usually talk about the three pairs of quarks: Up/Down, Charm/Strange, and Top/Bottom. For each of these quarks there is a corresponding antimatter-quark (anti-quark.) Quarks have the unusual characteristic of having fractional electric charge of either 2/3 or -1/3, unlike the -1 charge of an electron and the +1 charge of the proton.