i read the GPL today. didn't understand it much though...
OK. A simple explanation.
When licensed under the GPL, an application has its freedom protected by copyright laws.
First of all, you have no right to use any software, in any way, unless you either own the copyright or have been granted a license by the copyright owner. Now, the key to the GPL license is that you have no other rights to use the software. You either agree to the GPL, or you find yourself another solution.
The aim of the GPL is to promote the re-use of good software code, to protect the freedom of all users to to use the software for any purpose, to modify it if they need to, and to protect their freedom to share those modifications.
The GPL allows you to redistribute the software freely (including the freedom to sell it), and it allows you to modify the software and make derrivative works, which you can also distribute freely. It allows you to do this, provided you accept some other provisions. First of all, if you redistribute the application you must
also distribute (or be prepared to distribute on request) the source code, in a machine-readable format, for no more than the cost of the medium.
If you decide to modify the application, or make your own application either based on or including parts of it, you may also distribute your modified application, but you may only distribute it according to the terms of the GPL. Basically, GPL software can only be used in the production of more GPL software, except by the original copyright owner.
The GPL is not a totally free license. It can't be, because it has to have the power to protect the rights of others. You're not
free to pass other people's work off as your own. You're not free to make modifications and pass those off as the original application. You're not free to make proprietory changes, and then to distribute your modified work without distributing the source code for your changes.
Notice that the prohibitions only apply to redistribution. You don't need to release source code for your changes if you don't plan to share your changes with anybody else.
That's about it. Remember, this is only a clarification - the details are on the license itself, not my post, and I could easily have messed up. This should give you the gist of it, though. If you have any more questions, feel free to grab me on IRC, or PM me. Alternatively, talk to your local free software advocacy group (which includes most Linux user groups).