<p>When I type 'from' (in a <a href="http://en.wikipedia.org/wiki/Language_Integrated_Query" rel="nofollow noreferrer">LINQ</a> query) after importing <a href="http://msdn.microsoft.com/en-us/library/system.linq.aspx" rel="nofollow noreferrer">System.Linq namespace</a>, it is understood as a keyword. How does this magic happen?</p>

<p>Is 'from' a extension method on some type?</p>

"from" is a language keyword (just like "if" or "foreach").
You don't even need to import System.Linq to use "from", but you need to use 3.5 framework.