The market leaders when it comes to Thin Client software are Citrix Systems and the Microsoft Corporation.
Microsoft Remote Desktop Services, known as RDS for short, provides the engine on Windows Server 2008 R2 based systems to allow access remotely.
On older versions of Windows Server including Windows Server 2008 R1, Windows Server 2003 and Windows 2000, Windows Terminal Services is used.
Windows Terminal Services, known as WTS for short, is in essence the same as RDS and Microsoft decided to rename WTS to RDS in their Windows Server 2008 R2 version. The latest version of Windows Server is Windows Server 2012 and also includes RDS.
They've also made a few improvements and enhancements. People still use the words RDS and WTS interchangeably in the real world.
Windows Terminal Services and Remote Desktop Services have been developed in conjunction with Citrix Systems.
Originally Citrix Systems licensed the code from Microsoft for their Windows Server product (NT3.5) and redeveloped this. Citrix Systems released this redeveloped version of Windows Server as Citrix WinFrame.
When the next version of Windows Server was released, NT4.0, Microsoft decided not to give the code to Citrix Systems for them to develop their own version.
Instead Microsoft and Citrix worked together to create Windows Server NT4.0 Terminal Server Edition. Citrix Systems kept some of their technology outside of this joint project, which allowed them to develop their Citrix MetaFrame product.
Citrix XenApp is the latest version of Citrix's thin client product line. Just as with Citrix Presentation Server and its predecessor Citrix MetaFrame, all have been developed outside the joint development work Citrix does with Microsoft with its RDS and WTS products.
That is, Citrix Systems works with Microsoft to help develop their WTS service and later RDS, whilst Citrix Systems gets to keep certain functionality they've developed for themselves. This functionality has become Citrix MetaFrame, Presentation Server and XenApp.
Think about RDS and its predecessor WTS as a car which is jointly developed by Microsoft and Citrix. Now think of Citrix also developing some enhancements to this car such as a new fuel injection system, better suspension and a supercharger.
All the products Citrix develops in this analogy need the car to function, as you wouldn't expect a fuel injection system, suspension or a supercharger to work on their own without a car?
The car without these additional components can still work, doing what it was designed for. Just like Microsoft's RDS (WTS for older versions of Windows Server) can still work without needing any Citrix components to function and just like the car analogy, will get you where you want to go.
If you want an improved experience through increase power and improved handling, then the car could have the better suspension added, along with the improved fuel injection system and supercharger.
This is exactly how the XenApp (including previous versions called MetaFrame and Presentation Server) are positioned. They provide an improved experience for people using and managing their Windows Server and RDS (including WTS) components.
The latest incarnation of the Citrix thin client products is Citrix XenApp version 7.5, which has been developed specifically for Microsoft Windows Server 2012. Many organisations are still using previous versions of XenApp such version 4.5, 5.0 with many also using XenApp 6 and 6.5.
The XenApp product line originally started out as Citrix WinFrame, which became MetaFrame, then Presentation Server and is now XenApp, as shown below in screenshot 6 'Citrix Thin client history' below.
There are many reasons why Thin Client technologies such as Citrix are popular, not only because Citrix is a great company which has the vision to make great products but because Citrix actually develops products which organisations need.
Citrix have been very clever in the way they've brought out other smaller companies, which has not only opened up new markets for them, but enhanced their already excellent product range.
Technology aside, one of the main reasons for using Citrix is cost especially Total Cost of Ownership, known as TCO for short. You can find more about TCO in Appendix A of this course.
Citrix Systems develops Thin Client products such as XenApp.
Microsoft RDS (formerly Windows Terminal Services) provides the engine for Thin Client type computing.
Citrix XenApp works with Microsoft Remote Desktop Services (Windows 2012, Windows 2008 R2) or Windows Terminal Services (Windows 2008 R1, Windows 2003 and Windows 2000)
Fat clients run applications locally that is, on the computer being used by the user.
Thin clients access applications being run remotely and are not involved in the actual running of the application.