IANAL, but my understanding is that a derivative product (something built on top of CI2 and distributed) needs to credit EllisLab. A website that uses CI2 does not need to give such credit, so long as you are not distributing your code.
With the MIT license and CI3, you can basically do what you want with the framework, so long as you retain the copyright notice and don't claim the framework as your own.
If I am mistaken, I trust someone will jump in and set me straight!
