The following is the result of a bit of research I undertook today so as to establish a basis for the discussion of the lower-level topics related to the fundamentals whereof reality is constructed, wherein we usually find discrepency in fact when read from these forums. I am as guilty as any one of us. I did not write this myself as such, it is more or less a plagiarization from various informed sources and might even be outdated in parts, particularly with respect to M-theory (strings), however I have included only that which applies to general discussion and this bodes no serious transgression of contemporary thinking in my view, nor does it infringe on intellectual copyright since the knowledge upon which the sources are based is already in the public domain. This not only assures the accuracy of the information contained herein, it also frees me from committing completely to the facts as stated, since I did not write them.
In "particle physics", a particle is a subatomic object with definite mass and charge. Particle and anti-particle have identical mass and spin but opposite charges. Some electrically neutral bosons (force particles) are their own anti-particles.
The energy unit of particle physics is the electron volt (eV), the energy gained by one electron in crossing a potential difference of one volt. To get an idea of scale, if the protons and neutrons in a Helium atom were 10 cm. across, the electron would be .1 mm in size, and the entire atom would be 10 kilometres in diameter!
Electrically charged particles interact by exchanging photons. In strong 'color' interactions quarks interact by exchanging gluons. As quarks and gluons move apart the energy in the 'color' charge between them increases. This energy is eventually converted into additional quark and anti-quark pairs. These are then seen to combine into hadrons: mesons and baryons. Neutron beta decay results in the production of a proton, electron and an anti-neutrino via a mediating W- boson.
There are about 120 types of baryons, fermionic hadrons composed of three quarks, or three anti-quarks. The proton is the only baryon that is stable in isolation. Its basic structure is two up quarks and one down quark. The picture of a proton as made of three quarks is a gross simplification. For example, we know from measurements that in a high-momentum proton only about half the momentum is carried by quarks, the rest is carried by gluons.
String theory can be considered a particular kind of particle theory, in that its modes of excitation correspond to different particles. All these particles, which differ in spin and other quantum numbers, are related by a symmetry which reflects the properties of the string. The fundamental difference between a particle and a string is that a particle is a 0-dimensional object in space, with a 1-dimensional world-line describing its trajectory in spacetime, while a string is a (finite, open or closed) 1-dimensional object in space, which sweeps out a 2- dimensional world-sheet as it propagates through spacetime.
A string can be either open or closed, depending on whether it has 2 free ends (its boundary) or is a continuous ring (no boundary), respectively. The corresponding spacetime figure is then either a sheet or a tube (and their combinations, and topologically more complicated structures, when they interact).
Strings were originally intended to describe hadrons directly, since the observed spectrum and high-energy behavior of hadrons seems realizable only in a string framework. Certain string theories can thus be considered alternative and equivalent formulations of QCD (Quantum Chromo Dynamics), just as general field theories can be equivalently formulated either in terms of "fundamental" particles or in terms of the particles which arise as bound states. Due to matters of scale, a string formulation, where mesons are the fundamental fields, may be unavoidable. Thus, strings may be important for hadronic physics as well as for gravity and unified theories; however, string models seem to apply only to the latter, since they contain massless particles.