Please read these few comments since they may help you in better understanding of this document. They will also help you to further explore the topic of GUI automation.
I am using a term "Win32" to refer to Windows operating system family : windows 98/NT/2000/XP.
"Win32::GuiTest" refers to perl library.
If you read a description of a function and you find a sentence "This is a simple wrapper around Win32 function" that means that you can find more details on how the function works on Microsoft's MSDN. I intentionally refrained from providing a link for a function description, since it tends to change every 1-2 years. But if you enter www.msdn.com and make a search for a function, for example "SetForegroundWindow", you should get API description as a first result.