There is no such thing.
Citrix is Terminal Services:
More than one user in the box at one time.
Cool thing: TS reuses common code among sessions:
100 people running Word?
one copy of winword.exe in ram
voila: a citrix app

Citrix simply has more management features than TS
and costs ALOT more.
TS is free in all servers post 2000
Citrix has remote printing done right...