GPL just means that you can do what you want with it with very few restrictions. If you want to create a CMS around phpBB3 you're by all means allowed to, even if you don't link back to phpBB or anything. If, however, you want to try and sell your code, you have to 1) leave the copyright within the files intact and 2) give your users the same freedom you got. ie you can sell it, but you can't prevent your users from distributing it for free or forking it themselves.
If you don't want to distribute your code that's fine as well - you don't need to. You get a lot of freedoms with open source software.
It's a very beautiful concept