Usually, if charge is described as just a number, it is in units of e, or the charge of a proton, which is approximately 1.6x10-19 Coulombs (also the absolute value of the charge of an electron). So, protons have charge +1, and electrons, -1, using units of e.
The charge of an atom or composite particle is found by adding the charges of its protons and electrons (since neutrons are electrically neutral). Therefore, the charges of such particles are integer values.
However, there are particles with fractional charge. It turns out that protons and neutrons are composed of particles called "quarks." These quarks, which come in different "flavors" (up, down, charm, strange, top, bottom) make up certain particles. They have fractional charge. Up, charm, and top all have fractional charge of +2/3, while down, strange, and bottom all have a charge of -1/3. Protons are composed of two up quarks and one down quark, so the total charge is +1. Likewise, neutrons are composed of two down quarks and one up quark, so the total charge is 0.
Quarks are confined to the particles they compose. This is, appropriately, referred to as "confinement." This is why we don't observe quarks--and therefore their fractional charges--outside their composite particles (such as protons and neutrons).
-- Natasha S.
(published on 01/25/11)