Intended for computer game development, Microsoft DirectPlay is a media-independent networking API that provides networking services at the transport protocol and session protocol levels. *DirectPlay's media independence means that DirectPlay sessions can be run on TCP/IP networks, IPX networks, and over directly connected modems and serial cables. DirectPlay's media independence also allows it to be extended in the future to support new standards and protocols as they become widely deployed. Also included in DirectPlay are the DirectPlay Lobby APIs, which allow applications to register themselves with DirectPlay so that other applications can easily launch them and pass them network addressing information. This allows you to write games that can be immediately launched by a number of different matchmaking services and utilities, without having to write special code for each launcher.
* [Source: MS Knowledgebase]