The actual arrangement of the atoms around the central atom in a molecule.
In order to find this we need to consider the number of regions of electron density (electron pairs or unpaired single electrons in some cases) around the central atom. The electrons will repel as far apart as possible as they have the same charge.
The valence shell electron pair repulsion theory (VSEPRT) gives us a means of working out the shapes of molecules and ions.
For a fuller account see VSEPRT
Molecules in three dimensions
Click here and when the viewing screen opens right click on the image to see the options.
note: the MDL Chime plug-in must previously be installed (download here)
This model explains the tetrahedral geometry of carbon and other atoms.
The electron structure of carbon is 1s2 2s2 2p2 suggesting that it should only be able to form two bonds (using the two singly occupied orbitals). However it is known to make four single bonds in many compounds and indeed never forms just two bonds. This can be explained by hybridisation - the mixing of atomic orbitals producing degenerate orbitals used for bonding.
When a particular molecule can be represented as several different Lewis structures is is generally not actually any of these, but a hybrid (mixture) of all of them. This can be represented either by using delocalised electrons, or through resonance (where each possible structure is drawn and the actual state 'resonates' between them. The delocalisation of these pi electrons (which is effectively what happens) makes the molecule more stable (as evidenced by lower energy) and gives the bonds a shorter length than would be expected.