A GPL (General Public License) theme is one which is completely free, as well as which can be modified by anyone. You can download it and make any changes to it. You are not bound to keep authors details tagged with the theme.
A CCL (Creative Common License) theme is also free to download and use. However, you are restricted to modify the theme or a part of it. Some part or the complete theme is in encrypted format. Also, you must tag Authors details with the theme (generally in Footer).