An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.
An anion, meaning "up", is an ion with more electrons than protons, giving it a net negative charge (since electrons are negatively charged and protons are positively charged). Conversely, a cation, meaning "down", is an ion with more protons than electrons, giving it a positive charge.
An ion consisting of a single atom is a monatomic ion. If it consists of two or more atoms, it is a polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions. When writing the chemical formula for an ion, its charge is written as a superscript "+" or "-" (depending on if the ion is positive + or negative -) following a number indicating the difference between the number of protons and the number of electrons. The number is omitted if it is equal to 1.
If an ion contains unpaired electrons, it is called a radical ion. Just like neutral radicals, radical ions are very reactive.