The structure of matter has been of interest since the times of the ancient Greeks, when Demokritos proposed that the world was built up of elementary atoms (from the Greek for indivisible). Only in the last century was the existence of atoms seriously considered, and indeed only in this century was the physical reality of atoms finally accepted.
Fortunately for us, the atom is not indivisible.
We know now that the atom consists of a small, but massive, positively
charged nucleus which binds a number of
negatively charged electrons
in its electric field. This positive nuclear charge determines the
chemical
properties of the elements. Thus an atom has Z electrons each having
a charge -e, with 
coulombs. The nucleus
has a charge +Ze. Z is the atomic number which
characterises an element.
The electron is, as far as we know, an elementary particle; i.e. it has no substructure. But the nucleus is not elementary; it is built up of constituents, and the interactions of these constituents and the structure they lead to is the subject of this course.
The nucleus consists (largely) of protons and neutrons (collectively known as nucleons). A new force operates at the nuclear level to hold them together in the nucleus -- this is the (strong) nuclear force, a short range interaction which does not extend much beyond the nuclear boundaries.
But, again, the nucleons are not elementary particles. Nucleons are believed to be made of quarks held together by what is known as the strong force. The attribute that the strong force couples to is known as colour. (The name is essentially whimsical: think of colour as a more complicated version of charge). The quarks are distinguishable (`coloured') while the nucleons are `colourless'. The nuclear interaction is an exchange interaction indirectly related to the strong force. Quarks are not found as free particles, but only in `colourless' combinations. Quarks come in several varieties, the common sort in nuclei being `up' and `down' quarks.
Associated with this level of matter is yet another interaction, which manifests in nuclear physics as the weak (nuclear) interaction; by this, up and down quarks can transform into one another by the emission of an electrons and another elementary particle, the neutral and (almost?) massless neutrino (or their antiparticles).
Quarks are thought at this time to be elementary particles, like electrons and neutrinos. But, wait, a recent publication has claimed evidence for quark substructure...