|MadSci Network: Physics|
As you have correctly stated, quarks are the building blocks of more massive particles, which are called hadrons. Hadrons can consist of three quarks, in which case they are called baryons, or of two quarks, and then they are called mesons. Examples for baryons are the familiar protons and neutrons, which constitute nuclei, as you surely know (there are speculations that so-called `dibaryons' with six quarks might exist, but this is established neither theoretically nor experimentally). Quarks are - as far as we know today - elementary particles, i.e. they are not built up from anything else. They are `point-like'. Morever, quarks are subject to the strong interaction as well as to the electroweak interactions, and this is one of their main differences to leptons:
Leptons are also elementary constituents of matter, just like quarks. But as they are not subject to the strong interaction, their behaviour is quite different. For example, they can appear alone. No one has ever observed a single free quark - they always come in pairs or triplets, a property which is called confinement and which emerges as a result of the strong interaction. On the other hand, leptons can get along quite well alone. Consider for example ordinary atoms: The nucleus consists of nucleons, which in turn are made of quarks. But there is more to an atom: Electron are bound to it by the electromagnetic force! The electron is a lepton, one of the twelve that we know.
Quarks and leptons are fermions, whose defining property is that they have to obey the Pauli Exclusion Principle: No two fermions can be in the same state (which is for example the reason that atoms are `filled' with electrons from the bottom up: One single energy level with certain values for energy, angular momentum and spin can only inhabit one electron).
The forces which I mentioned above are mediated by so-called gauge bosons (bosons do not obey the Pauli Principle). For example, the strong interaction uses gluons as `force carriers'. Gluons are massless particles with no spin. In the most successful theory of elementary particles we have today, the `Standard Model', all interactions use bosons as force carriers. That does not mean that there are no bosons around which do not belong to any interaction: Mesons, which I mentioned above, are all bosons. They are massive, compound particles made of quarks. On the other hand, there are fermions which are not elementary: The neutron and the proton are the most prominent examples.
So you see that the world of elementary particles can be divided into matter constituents (the quarks and leptons) and force carriers (the gauge particles). The universe seems to be made in a way such that matter constituents are fermions and force carriers are bosons. But keep in mind that the terms `boson' and `fermion' exist quite independently from `quark', `lepton' and `gauge particle'. The former are just shorthands for certain important features of particles (concerning the Pauli Principle), the latter are real in a sense that I know exactly what I mean when I speak of a `red charm quark', for instance.
As to why those particles are there: Physicists have no clue! Some of the features of the perticle zoo can be deduced from very general principles (for example the fact that all force carriers are bosons), but other things must be put in by hand (for example the numbers and masses of the different kinds of quarks and leptons). A pragmatic point of view shared by many physicists is ``The theory works - why should we bother?''. This is of course not the opinion which one should favour, but there is a grain of truth to it.
To learn more about elementary particles and their interactions, you should take a look at The Particle Adventure. If you still have more nagging questions, feel free to submit another question to the MSN or e-mail me directly.
Try the links in the MadSci Library for more information on Physics.