You won’t find a basic answer here. They use much more involved mechanisms for detecting changes on the screen and sending them.
Check out how terminal svcs work –
http://technet.microsoft.com/en-us/library/cc755399%28WS.10%29.aspx
ideally you are hooking into the GUI and monitoring events, etc. much more advanced than simply screen scraping. If you want to look at less advanced code check out http://www.tightvnc.com
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solved Taking Screen shot continuously without slow down the PC – C#