An ion is a atom or molecule with a net electric charge. The net charge of an ion is not zero because its total number of electrons is unequal to its total number of protons.
When writing the formula for an ion, it's absolute net charge is written in superscript above the ion and then the sign of the charge is placed after (either a + or a minus). If the absolute net charge of the ion is 1 the value is omitted and just the sign is written.