Polarity

If the electrons are not distributed equally, the molecule is said to be polar. That is, the molecule has a negative end and a positive end. It has two poles and is polar; it has a measurable dipole moment.
Water molecule

The electrons are not distrbuted evenly so the water molecule is polar. The negative end of the molecule is the oxygen end. O is more electronegative than H and pulls the negative electrons toward itself. Also, there are two lone pairs around oxygen.

Water molecule

You can write a formula denoting the negative and positive end of the molecule. The sigma (with a plus or minus) above the symbol for the atom is the indicator for which end of the molecule is positive or negative.