What are the different types of ions?

Cations are formed when an atom loses electrons. One common example is the sodium ion (Na⁺) is formed when a sodium atom (Na) loses one electron. 

Anions are formed when an atom gains electrons. An example is  the oxide ion (O2-) which is formed when an oxygen atom gains two electrons, resulting in a negative charge on the ion. 

Polyatomic ions are small groups of charged atoms referred to as polyatomic ions. They have an overall charge due to the presence of excess electrons (negative charge) or a deficiency of electrons (positive charge). One example is a hydroxide ion (OH⁻), which is composed of one oxygen atom and one hydrogen atom covalently bonded, carrying a net negative charge.