Answer:
Polarity is caused by the difference in electronegativity (EN) of a compound. For example, water, H2O, is polar because oxygen's EN is much higher than hydrogens. Therefore, the electrons spend more time around the oxygen and less time around the hydrogen causing a slightly positive pole at the hydrogens and a slightly negative at the oxygen. Most polar molecules are not symmetrical (like water). Non-polar molecules like CH4 (methane) do not have poles.
Hope that helps
