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.

From the Citrix website

• Gives workers secure, easy and instant access to enterprise applications and information from anywhere, at anytime, using any device, over any connection."
• Enables IT staffs to manage heterogeneity by centrally consolidating applications, simplifying their deployment, management, monitoring and measurement.
• Ensures that the right people have access to the right resources to protect the security of enterprise information assets

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.

Citrix like Microsoft have their own qualifications network that is taught at many private training schools in NewZealand more information can be found at their website