Non-Polar Covalent Bonds
A non polar covalent bond has two electrons shared equally between two atoms. For example, in naturally occurring hydrogen gas two electrons are shared equally between two hydrogen atoms.
Polar Covalent Bonds
A polar covalent bond has unequal sharing of hydrogen atoms. This unequal sharing is caused by atoms with unequal electronegativity. The atom with greater electronegativity repels the electrons further.
An atom's polarizabilty can be measured as the ease of distortion of the electric field associated with an atom or a group. For example, a fluorine atom in a molecule holds its electrons tightly and is very non-polarizable. On the other hand, an iodine atom in the same molecule would be very polarizable since its electronegativity is much less than that of fluorine. The most electronegative atoms are in the upper right-hand corner of the periodic table. On the periodic table, electronegativity increases as you move up in a group or right in any period. As an example, the bond between carbon and hydrogen is not polar since they have electronegativities of 2.5 and 2.1 respectively. Some examples of bonds with high polarity are between hydrogen and oxygen, fluorine, chlorine or nitrogen.
The greater the difference in electronegativity between the two atoms, the stronger is the polarity of the bond.
Bond Dipoles
A polar bond consists of a positive and negative dipole. The negative dipole is the atom with the higher elctronegativity. The electrons are attracted to the more electronegative atom, the negative dipole. Dipole bonds lead to dipole moments. In a two atom molecule, the dipole moment points from the positive dipole to the negative dipole. In a molecule such as water the negative dipole is the oxygen atom, the dipole moment is located between the two hydrogen atoms.
Some things that are polar:
salts, detergents (one portion is polar, the other non-polar), carbondioxide, water, alcohols
Some things are not polar:
benzene, fats, ether, propane
This is an index of links that will help you further understand polarity.