It is Hydrogen bonds.
An intermolecular force holds molecules together. (Do not confuse it with bonds which hold atoms together).
Now since water is H2O, the molecule is polar due to the big difference in electronegativity between the O and H atoms. Electrons are hence pulled further to the O side in the bond. Hence it is very strongly attracted to other nearby water molecules and these strong intermolecular forces are called hydrogen bonds.
Here's a link to a more detailed answer posted by another contributor: