Read about the sources on the website:
http://apps1.eere.energy.gov/buildings/energyplus/weatherdata_sources.cfm
Or read the whole weather information there.
Also is explained in the Auxiliary Programs document as well as this
Drury B. Crawley. 1998. " Which Weather Data Should You Use for Energy Simulations of Commercial Buildings?" ( in ASHRAE Transactions, pp. 498-515, Vol. 104, Pt. 2. Atlanta: ASHRAE.