In water and ice there are two types of bonds, covalent bonds within the water molecules, and hydrogen bonds between the molecules. Hydrogen bonds are weaker than the covalent bonds.
As the water freezes, the hydrogen bonds start to take a greater part in the bonding, which orders the molecules into a regular crystal structure. This regular structure is not as closely packed as the random liquid structure, which means the water expands slightly, decreasing its density.
