DirectPlay is a network communication library that is part Microsoft's DirectX APIs (although has been largely replaced by Games For Windows - Live platform). DirectPlay provides an interface between applications (games) and communications services. DirectPlay, then, can provide a link between players over the Internet, regardless of what actual hardware is used to establish the link (modems etc.), and it resolved many previous connectivity issues that network gaming came up against before it. In 2004, Microsoft revealed that DirectPlay would be deprecated in favor of the Games for Windows - Live technology that was already in use with the Xbox console at the time. DirectPlay is deprecated with the Windows Vista release.
Other DirectX related glossary:
DirectDraw
Direct2D
Direct3D
DXGI
DirectInput
DirectPlay
DirectPlay Voice
DirectSound
DirectSound3D
DirectMusic
DirectX Audio
DirectX Media
DxDiag
DirectX Media Objects
DirectSetup
XAudio2
