Difference between Polar and Nonpolar Molecules

A molecule is composed of elements which are bonded by chemical bonds. 
A chemical bond can be either ionic or covalent in nature. 
An ionic bond is an electrostatic force of attraction between cation and anion, therefore it formed by complete transfer of electrons.
While covalent bond is form by equal sharing of electrons between bonded atoms. 
Due to the presence of positive and negative poles in ionic molecule, they are polar in nature and consist of some dipole moment whereas molecules having zero dipole moment are said to be nonpolar molecules like covalent molecules.
The charge separation in a chemical bond due to unequal sharing of the electrons is known as a bond dipole.

As the difference in electro negativity between the bonded atoms increases, the magnitude of bond dipole also increases. 
The overall dipole moment of a molecule is the result of the vector sum of all the bond dipoles present in it. 
In a covalent bond between two similar atoms, the bonding electrons lie midway between nuclei of bonded atoms because both the atoms have same attraction for bonding electrons.

There is a equal sharing of electrons if and when we have same atoms coming in for bonding. 
These sharing becomes un-equal if and when we have unlike atoms coming together for bonding that results into the shift of electric charge from one partner to the other. 
The more electronegative an element is the more it starts attracting electrons.

When two atoms are bonded covalently their abilities of attarcting electrons are almost same and this leads to a situation where there is equal sharing of bonding electrons and will be considered as a non polar covalent bond.

Bondings between H atoms or F atoms and forming H2 and F2 are basically between same atoms  and thus results in non polar molecules.

When two atoms are bonded with different abilities to attract electrons or with different electronegativity the resultant covalent bonding exhibits polarity. 

So definitely the covalent bondings between hydrogen-fluorine, nirogen-oxygen, sulfur-oxygen and even water where we have hydrogen-oxygen are nothing but polar covalent bondings.

The un-equal sharing of electrons makes the more electronegative end of the molecule more negative than the partner and the bonding is drawn more towards the electronegative partner.

The polar bonds are plotted between the extremes of both non-polar covalent bonds and the ionic bonds.

In a covalent bond when we have same atoms combining, there is no charge separation or what we mean is that the negative charge of the elctrons are evenly distributed over the bond.

In ionic bonds there is a complete separation of the charges and this is not visible in polar bondings where it is in between these two extremes.

For any kind of polar molecules the intermolcular forces would be acting in-between the +ve end of one polar entity and the -ve end of the adjacent polar entity. In molecules like sulfur di-oxide both the entities are polar.

In this molecule the partially negative region of one of the entities would be attracted to the adjacent entities partial positive region. 
Therefore the molecular orbital of covalent bond is symmetrically distributed around atoms.

Such type of covalent bonds is known as nonpolar covalent bond and molecules are known as nonpolar molecule like H2, Cl2, O2.

In diatomic molecule with two different atoms, the polarity depends upon the electro negativity difference of both bonded atoms.

Like in Hydrogen Chloride Molecule (HCl), the bonding electrons get attracted towards more electronegative chlorine atom and create polarity in molecule. 
In polyatomic molecules the dipole moment of molecule depends upon the orientation of various bond dipoles.

In the symmetric molecules the net dipole moment of the molecule is zero due to cancellation of bond dipoles.
Like in Carbon dioxide (CO2) molecule, the bond dipole of both carbon-oxygen bonds gets cancelled with each other and makes the molecule nonpolar.

Whereas in water (H2O) molecule; the bond dipole of both oxygen-hydrogen bond cannot be cancelled due to bent geometry of molecule hence the resulting dipole moment of water molecule is 1.83 D.

In the similar way, the resulting dipole moment of BF3 and CCl4 is zero as the bond dipoles of bonds give a net sum of zero due to the resultant of any two is equal and opposite to the third.
Thus both molecules are nonpolar molecules. However the bond dipoles of three nitrogen-hydrogen bonds in ammonia molecule (NH3) do not cancel each other. 

Hence the molecule has a net dipole moment 1.49D and polar in nature.
Overall; all ionic molecules are polar in nature while covalent molecule can be polar or nonpolar.  

Polar molecules have a charge separation with a certain value of dipole moment while non-polar molecules have zero dipole moment. 
Polar molecules get are attracted by electrostatic attraction while nonpolar molecules remain unaffected.

 Some common molecules with their polar and nonpolar nature are as follows;