A hydroxyl is a chemical functional group containing an oxygen atom connected by a covalent bond to a hydrogen atom, a pairing that can be simply understood as a substructure of the water molecule. When it appears, it imparts to chemical structures some of the reactive and interactive properties of the -OH of water (ionizability, hydrogen bonding, etc.). The neutral form of the hydroxyl group is a hydroxyl radical. The anion form, (OH−) is called the hydroxide anion; it bears a single negative charge largely residing on the more electronegative oxygen.
Read more about Hydroxyl: Hydroxyl Group, Hydroxyl Radical, Lunar and Other Extraterrestrial Observations