In the Valance Bond Theory of molecular joining, two different elements join together by sharing electrons. In essence, they overlap each other and the overlapped area, the area where the shared electrons are held, is the strongest point of the bond. This is very similar to a Venn Diagram. It was commonly thought that this shared area was localized, also, but that's turned out to not be quite true.
In the Molecular Orbital Theory, bonding occurs because of the location of only one electron. Electron don't belong to the original elements but to the new molecule. As SparkNotes.com says. . .
"Molecular orbital theory holds, as its name suggests, that electrons reside in molecular orbitals that are distributed over the entire molecule."
I guess this method is more accurate to more accurately describe the process that takes place when elements join together to make a compound. They both lose their original identity and become soething altogether new.