Could someone please explain? I couldn\'t find anything on the internet, everything talks about how to go about it in some way, but nothing says exactly what it is.
Maybe some context will help.
Think about something like browsing stackoverflow. There is the code off the browser itself that can do anything on your computer (delete files for example ) and there is the javascript code of the site. The javascript code can't do anything to your computer except the nifty fade effects and some other display magic.
.net has the ability to provide such distinction between local application and remote ones. You can write application that will run on the local computer and will be able to do anything to the local computer. And this application can have a feature that download extension dll from the internet. Those extension will be able to do calculation and manipulate the display. But the code inside them want be able to delete files. Because it is not trusted.
The details are wrong but this is the idea ( as far as i understand it).