If electrons are not always evenly distributed between bonds, it would make sense that the electrons are not evenly distributed through the entire molecule.
Polar Molecules - molecule where the shared electrons are not evenly distributed throughout the molecule. These molecules are called dipoles since you have a partially negative and partially positive areas of the molecule.
Non-polar Molecule - molecule where the shared electrons are evenly distributed throughout the entire molecule.
Key to figuring Molecular Polarity -Assume Polar, prove Non-polar
Since it is easier to have the electrons unevenly distributed throughout the molecule, polar molecules are statistically seen more often.
Molecular polarity is a function of Bond Polarity and Shape of the Molecule. There are certain shapes (linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octehedral) where polar bonds will cancel themselves out (due to the symmetry in the shape) so it these bonds will act like non-polar bonds.
Rules for Determining Molecular Polarity
Find the bond polarity of all the bonds in the molecule.
Determine the geometry of the molecule.
If you have all non-polar bonds, you have a non-polar molecule
If you have all polar bonds, you have a polar molecule , except for molecules that have symmetry (trigonal planar, linear, tetrahedral, trigonal bipyramidal, octahedral) since the polar bonds will cancels themselves out due to symmetry to form a non-polar molecule.
For compounds with both polar & non-polar molecules, need to check to see if polar bond can themselves out due to symmetry to give non-polar molecules.
For oil, grease, gasoline and fats (these compounds have CxHy), these compounds are all non-polar molecules since the C-H & C-C bonds are all non-polar.