This FAQ covers the Remote Desktop Protocol (sometimes referred to as Terminal Services or Remote Desktop Client), Microsoft's protocol for remotely accessing Windows computers, both desktops and servers. It covers what the protocol is, details about its features, available software, and how to accomplish tasks like enabling multiple simultaneous remote desktop sessions on XP/Vista/Windows 7.
Wikipedia has a good overview of the history of Microsoft's Remote Desktop Protocol.
The server software, currently called "Remote Desktop Services", was originally called "Terminal Services", and first included in Windows with the Windows NT 4.0 Terminal Server Edition, released in 1998. RDP has been included in every version of Windows since then, although some basic Windows editions removed support. The client software was originally called "Terminal Services Client", but is now referred to as "Remote Desktop Connection".