Question

How does polarity relate to electronegativity?

Answer 1

Electronegativity is the measure of an atom to attract electrons for bonding.
When a covalent bond (sharing) is created between two atoms (oxygen and hydrogen) the atom (oxygen) with the greater electronegativity gets the BIGGER share of the electron pair.
This creates a partially negative charge at the oxygen and a partially positive charge at the hydrogen creating a polar bond.
Now the molecule ($H_{2}O$) will have a partially positive charge on the hydrogen atoms and a partially negative charge on the oxygen creating a polar molecule.

Note: The negative charge on the oxygen is not only because of its electronegativity but also because of the non-bonding pairs of electrons.