Terminal Services is a technology which Microsoft has been integrating in
various ways into products since NT4 Terminal Services Edition, at least.
In XP, Terminal Services technologies underpin 3 features: Remote Desktop,
Remote Assistance, and Fast User Switching.
XP Pro has all three of these features, XP Home Edition lacks Remote
Desktop.
Terminal Services in Server level products allow for much more complex
uses--ranging from background Administrative sessions to full-blown Terminal
Servers where multiple user sessions run on a single machine, allowing for
simpler administration and lower cost client hardware.