Hydrogen: Which in Greek means "water-former." At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustible diatomic gas with the molecular formula H2. It is the lightest and most abundant chemical element, constituting roughly 75% of the Universes chemical elemental mass.
HHO: Is the term used in short for "Oxyhydrogen". HHO gas is the product of electrolysis on H2O or pure water. Oxyhydrogen is a mixture of hydrogen (H2) and oxygen (O2) gases, in a 2:1 molar ratio, the same proportion as water.