An ion is an electrically charged atom or molecule. It has a charge from losing or gaining electron(s). An anion has a negative charge - it has gained electron(s), a cation has a positive charge - it has lost electron(s). All elements want to gain a full outer shell of electrons.
Ions are written the same as the atom or molecule, with a superscript of the charge on them next to them, e.g. H+.