The fact that the oxygen end of a water molecule is negatively charged and the hydrogen end positively charged means that the hydrogens of one water molecule attract the oxygen of its neighbor and vice versa. This is because unlike charges attract. This largely electrostatic attraction is called a hydrogen bond and is important in determining many important properties of water that make it such an important liquid for living things. Water can also form this type of bond with other polar molecules or ions such as hydrogen or sodium ions. Further, hydrogen bonds can occurr within and between other molecules. For instance, the two strands of a DNA molecule are held together by hydrogen bonds. Hygrogen bonding between water molecules and the amino acids of proteins are involved in maintaining the protein's proper shape.
This picture represents a small group of water molecules. Hydrogen bonds between unlike charges are shown as lines without arrows on the ends. The double arrowed lines represent the fact that like charges repell each other. Both hydrogen bonds and the repelling forces balance each other and are both are important in determining the properties of water.