You can add themes to change the look and feel. You can add modules to add functionality. But you can also add providers to login using social media or your credentials of your office network.
But it doesn't stop there. Like many crafts(wo)men, developers may have developed their own ways of creating awesome things. If you just want to leverage the users/roles/permissions of DNN but don't want any of the other stuff, you can do that too. DNN has an extensive API to use whatever you need and whenever you need it.