A molecule is defined as a sufficiently stable, electrically neutral group of at least two atoms in a definite arrangement held together by very strong (covalent) chemical bonds. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules and biomolecules.
A molecule may consist of atoms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are generally not considered single molecules.
For more, visit the Wikipedia entry for molecule, URL: http://en.wikipedia.org/wiki/Molecule