Trans Cranial Technologies offer this manual which explains the International 10/20 system for describing the location of scalp electrodes. 10/20 System Positioning (pdf) See Also: Wikipedia, Electrode Positions

The system is based on the relationship between the location of an electrode and the underlying area of cerebral cortex. The numbers ‘10’ and ‘20’ refer to the fact that the distances between adjacent electrodes are either 10% or 20% of the total front- back or right-left distance of the skull.

Each site has a letter to identify the lobe and a number to identify the hemisphere location.