GPL linking exception

Some projects, like GNU Classpath distribute code under under the terms of the GPL but with the following clarification and special exception.

Linking this library statically or dynamically with other modules is making
a combined work based on this library. Thus, the terms and conditions of the 
GNU General Public License cover the whole combination.

As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent modules,
and to copy and distribute the resulting executable under terms of your
choice, provided that you also meet, for each linked independent module,
the terms and conditions of the license of that module. An independent
module is a module which is not derived from or based on this library.
If you modify this library, you may extend this exception to your
version of the library, but you are not obligated to do so. If you do
not wish to do so, delete this exception statement from your version.

Thanks to this exception a software licensed with it can be used to run, create and distribute all kinds of Open Source and Closed Source applications.