What causes bond polarity?

An electronegativity difference between the atoms in a molecule.

An difference between the atoms in a molecule causes bond polarity. For example a H2O molecule has the structure of H-O-H and although you can't see it from here. You need to keep in mind that Oxygen is more electronegative than hydrogen, meaning it has a stronger tug on electrons. So the electrons in each of the H-O and O-H bonds are much closer to the O then Hydrogen. And as a result the Oxygen is more negative due to this and has a partial negative charge on it. The Hydrogens are more positive due to the electrons being farther away and thus has a partial positive charge on them.