What are covalent bonds?

Covalent bonds are those in which electrons are shared by the atoms, and this sharing results in an attraction between the atoms to form a molecule. A powerful bond tightly holds together the elements, forming a new substance. Whenever possible, the electrons in a covalently-bonded molecule are shared in such a way so that eight electrons surround each element. This bonding completes the outer shell electrons of all the participating atoms and fulfills the octet rule of electron shells.

Remember that a full outer shell of electrons is electrically stable, and this configuration is "sought" by atoms, which, by their natural composition, lack a full (complete) outer electron orbital. Covalent bonds are very strong because this sharing of electrons is very powerful and produces new substances with properties entirely different from the properties of the original elements.