GPL
From RemObjects Software
The GPL is a license by the GNU Project meant as a way to release software as free software and making sure that it stays that way. It does this by requiring that everything that directly uses GPL based code, also has to use GPL or a GPL compatible license, if it's redistributed.
The GPL FAQ has a list of common questions about the GPL license. Notable usage of the GPL license is by the Linux kernel and most tools included with Linux distributions.
LGPL
The Lesser GPL is a derivative of the GPL with an exception for linking against LGPL based libraries. This makes it possible for commercial libraries to use LGPL libraries while still keeping open the redistributed changes to the original library.
