Chemical Stability and Electronic Configuration

The noble gases have 8 electrons on the outermost energy level, except He (2e). Such rule is applied to all compounds. When the molecule formed, it changes its electron configuration to full saturation: 8 electrons “Octet rule”. It is done either by losing, gaiting or sharing electrons.

source:

http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/covalent_bond/covalent_bond.html

What is the Octet rule?