H2O is the chemical formula for dihydrogen monoxide, which is water. It is the combination of two hydrogen atoms bonded to a single oxygen atom.
H2O is the most abundant compound on the surface of the Earth. Approximately 70 percent of the planet is covered in water, though not all of it is H2O. The majority of the water on the Earth's surface is seawater, in which various other compounds including NaCl (sodium chloride, or salt) are dissolved. These compounds do not bond to the H2O molecules and so the chemical formula does not alter, but seawater is not strictly pure H2O.
Water is the only substance that can be found naturally on the Earth's surface in all three of its phases (liquid, solid and gas).