What is the difference between a polar molecule and a non-polar molecule? Can non-polar molecules be formed by polar atoms?

Polar molecules are those that have permanent dipoles. A permanent dipole occurs due to a difference in the electronegativity of the atoms involved in the bond. You might not be familiar with the term ´electronegativity`. This term reflects how strongly an atom pulls electrons.  Therefore very electronegative atoms pull electrons strongly. An example is Fluorine.  Finally, non-polar molecules can be formed by polar atoms. This is very commonly seen in O2 or  CO2. Their bonds are polar but they are in fact non-polar molecules.