Polar molecules (such as water) carry dipole charges (slight positives and negatives). This means they tend to be very good solvents and have higher boiling points than predicted. The polar ends of the molecule are created because the electrons floating around the molecule tend to move toward a certain element of the molecule (in order to complete its outer shell and bonding requirements). This element is said to have a higher electron affinity than the other elements of the molecule. This leaves for example the hydrogens in a water molecule slightly lacking in electrons (delta +ve) and the oxygen with a slightly higher electron density (delta -ve).
Polarity is determined by the electronegativity of the elements within the molecule. For example in water, Oxygen has a higher electronegativity than Hydrogen so the shared electrons within the molecule are drawn to the oxygen atom more than two hydrogen atoms. Since the negatively charged electrons have moved towards the oxygen it becomes slightly negative and the hydrogen slighlty positive.