Where is the information stated? In the license, naturally
Note the special handling of the trademark and your obligations in the software license. If you make cool additions to Moodle, when you give the software to your users/clients the GPL applies and you cannot stop your them from re-distributing them.
It is a nice and fair arrangement: you can take advantage of the efforts of the Moodle team... and viceversa! (In the general case, read below if you want to learn in detail.)
This will probably also help
In terms of how you can use Moodle, what business models work around software with this type of license, I would strongly recommend the classic "Open Sources" book http://oreilly.com/catalog/9781565925823/
I have the hardcopy, but you can read it online too for free. If you want to cut to the chase, chapter 11 is the most relevant for you today.
The "2.0" edition is a completely different book. Also good, but deals with other "problems".