In 1978, the Patent Cooperation Treaty (PCT) went into effect to streamline multi-country patent applications. Published patent applications filed through the PCT (as opposed to through an individual country) have the prefix WO. As additional patents get issued in individual countries (patent numbers with country prefixes), they will be added to the original WO "patent family", and certain databases will link all of these together by the PCT Application Number.
PCT is maintained by the World Intellectual Property Organization (WIPO), an agency of the United Nations. More information about WIPO can be found on the Other Countries tab above.
In order to better organize the plethora of patents being filled, patents are usually classified using one of three different systems: