Maria Spiropulu is a physics doctoral candidate at Harvard. Her response follows:
Let's start by defining matter. People have asked "what is matter?" for quite a long time. Democritus, the ancient Greek philosopher and mathematician, envisioned structure in the building blocks of everything and he called the basis for this structure an atom; he wrote, "nothing exists except atoms and empty space: everything else is opinion." At the atomic level, the world can be described in terms of the elements, including hydrogen, oxygen, carbon and the like.
As it turns out, though, atoms are not the fundamental constituents of matter. When we zoom closer into matter, by probing at smaller distances, the subatomic world unfolds. The closer we look, the stranger this world, the quantum world, actually behaves. We can not make a direct connection with it: at a small scale, objects do not behave like rods or balls or waves or clouds or anything we have ever directly experienced. But the quantum mechanics of this world does let us describe how atoms form molecules.
It also enables us to depict the "motion" of certain particles inside atoms. Indeed, atoms are made of electrons that whiz around the fixed protons and neutrons in their nuclei, which are made of quarks. These particles all interact with each other by means of "force messenger" particles, such as photons, gluons, W's and Z's. Based on the attributes of these particles, we assign them identification numbers, or quantum numbers. And by means of symmetries and conservation laws involving the quantum numbers of the particles, we can describe their interactions. Examples of such numbers are charge and intrinsic angular momentum, or spin.
If a is any particle and this particle has no attributes other than linear and angular momentum (which include energy and spin), then a is its own anti-particle--one of the constituents of antimatter. For example, the photon is its own anti-particle. If a particle has other attributes (such as an electric charge Q), then the anti-particle has the opposite attributes (or a charge of -Q). The proton and neutron have such attributes. In the case of the proton, its positive charge distinguishes it from the negatively charged anti-proton. The neutron--although electrically neutral--has a magnetic moment opposite that of the anti-neutron. Protons and neutrons have another quantum number called the baryon number, which also has the opposite sign in the corresponding anti-particles.
The operation of changing particles with anti-particles is called Charge conjugation (C). Particles and anti-particles have the exact same mass and equal, but opposite charges and magnetic moments; if they are unstable, they have the same lifetime. This period is called the Charge Conjugation-Parity-Time (CPT) invariance, which establishes the fact that if you interchange particles for anti-particles (C), look in a three dimensional mirror (P) and reverse time (T), you cannot tell the difference between the them. The most stringent tests of CPT to date are measurements of the ratio of the magnetic moments of the electron and positron to two parts in a trillion (R. Van Dyck, Jr. and P. B. Schwinberg, University of Washington,1987) and measurements of charge per mass of the proton and antiproton--found to be 0.999,999,999,91 to 90 parts per trillion (G. Gabrielse, Harvard, 1998).