Citrix is a suite of applications that allow applications to be run on hardware and or OS's that do not support them.
Companies often use it to run appications on Thin Clients and/or older hardware.
Citrix suite is broken into 2 main groups which they call MetaFrames 1.8 and XP.
MetaFrame 1.8 is pretty much obsolete as it was for Windows NT4 and early versions of Unix
MetaFrame XP is the current release and is designed to run on Windows 2000, Windows 2003, Solaris, HPUX and AIX.
Currently MetaFrame XP does not support Linux and there is no details released to say if it ever will, there is a Citrix client for Linux that allows you to make a connection to a Citrix server and load applications on your Linux desktop.