See dual layer.

Webopedia Quick Reference: DVD Regional (Country) Codes
DVD movies are usually coded with what is called a country or regional code or a zone lock. The three names, however, mean the same thing. DVD movies are given a regional code and the code on the DVD movie must match that of the DVD player to be viewed.

Webopedia's Did You Know...DVD Formats Explained
The crucial difference among the standards is based on which standards each manufacturer adheres to. Similar to the old VHS/Beta tape wars when VCRs first hit the markets, different manufacturers support different standards.