In chemistry, H2O is the formula for water. H2O represents one water molecule, which is made up of two hydrogen atoms and one oxygen atom bonded together. When properly written, the "2" after the "H" should be in subscript.
The bonds that hold the hydrogen and oxygen atoms together in an H2O molecule are called covalent bonds. Covalent bonds are very strong, unlike weaker ionic bonds found in many other chemicals. Water is the most abundant molecule on Earth and covers approximately 70 percent of the planet's surface. The shape of the H2O molecule allows it to easily stick and clump together, which is why water has high surface tension.