Molecular Polarity
Molecular polarity results from an uneven distribution of charge throughout a molecule. In a polar molecule, dipoles do not cancel out, giving one end a partial negative charge and the other end a partial positive charge.
Water is a polar molecule (see image below; taken from Mr. Tang on SlideShare). The O–H bond is a polar bond because oxygen is more electronegative than hydrogen. This means oxygen has the greater ability to attract electrons to itself in a bond. In water, there are 2 O–H bonds. Dipoles form, resulting in an overall polarity.
Notice how the oxygen end has a partial negative charge.
Polarity allows water molecules to form hydrogen bonds.