A cation is an ion with a positive charge. Anions are ions with a negative charge.
An easy way to remember the difference is;
a ca+ion (the T becomes a plus sign)
a negative ion
When atoms gain or lose electrons, so that the number of electrons does not equal the number of protons in the nucleus, the imbalance in electric charge results in a ion. While it is technically possible for any atom to form either a cation or anion, in everyday life, the ionic charge is often determined by the element's location on the periodic table according to its number of valence electron (those on the outermost part of the atom). Generally speaking, atoms don't want to have partially-filled electron shells, so they'll gain electrons if that's the easiest way to fill the shell, or lose them if that's the easiest way to empty the shell out.
Metals usually form cations, and nonmetals usually form anions. Groups 1A, 2A, 3A, the D-block transition metals, and the F-block lanthanides and actinides form cations. Examples include hydrogen, magnesium, iron, aluminum and uranium. Groups 5A, 6A and 7A form anions - examples include nitrogen, sulfur and chlorine. Groups 4A and 8A tend not to ionize.
Cation is a positively charged atom.
Examples of cations: H, Pb, Au, C, U
An anion is a negatively charged atom.
Examples of anions: Cl, O, S