Writing a "Cross Loop"

[the swift]

I've recently just came across a program called CrossLoop, http://www.crossloop.com/

I was looking at it and was wondering how to possibly WRITE a program that lets you take control of other people's computer.

Of course they'll have the program on their computer too, but I was simply wondering how to be able to, over the internet, do things on the other people's computer.

Thanks.