TastyIgniter is licensed under the GNU General Public License, version 3 from the Free Software Foundation. A copy of the license is included with every copy of TastyIgniter, but you can also read the text of the license here.2. Does the license cover just PHP, or everything?
The "source code" of a file means the format that is intended for people to edit. What that means depends on the file in question.
Yes. TastyIgniter extensons and themes are a derivative work of TastyIgniter Community. If you distribute them, you must do so under the terms of the GPL version 3. You are not required to distribute them at all, however.5. Can I sell a TastyIgniter extension, or a TastyIgniter theme?
Yes. However, you must distribute it under the GPL version 3, so those you sell it to must be allowed to modify it. See questions 3 and 4.6. If I write an extension or theme, do I have to give it away to everyone?
No. The GPL requires that if you make a derivative work of TastyIgniter and distribute it to someone else, you must provide that person with the source code under the terms of the GPL so that they may modify it under the terms of the GPL as well.
However, if your extension is of general use then it is often a good idea to contribute it back to the community anyway. You can get feedback, bug reports, and new feature patches from others who find it useful.7. Why is TastyIgniter a registered trademark? Does that mean it's not free?
The software itself is completely free, under the terms of GPL. However, the name TastyIgniter is a registered trademark "to create a level playing field for everyone interested in using the TastyIgniter trademark."