(H2O)2 is called a hydrogen-bonded complex.
The bonds are described as weak because their energies are small compared with a covalent bond. The hydrogen bond energy in (H2O)2 is only about 20 kJ mol-1, compared with the O-H bond energy of 460 kJ mol-1.
Hydrogen bonds are a key feature in the assembly of some molecules into large units, like the double helix of DNA.