@flo: https://stackexchange.com/ is a host for numerous Q&A Forums, of which StackOverflow.com is one – note the black bar across the top of the screen that says “Stack Exchange”. Therefore, when you create a Stack Exchange login, you are able to use that one login to access all the forums they host, including StackOverflow.
On the login page that you linked: https://stackoverflow.com/users/signup you can tell that you are creating a NEW login for Stack Exchange, by the fact that it has you enter the password twice, for confirmation.
Many – if not most – sites (including Fluther) request an email address so they can verify that you are a human and that the person with that email address genuinely wants to create an account on their site. They also use that email address to send you communications about their site and service, and you can often specify which types of emails you want to receive – if any. For a lot of sites, your username is your email address. However, it is up to the user to create a unique password for that site, and not give them the password that you use to access that email.
As a traveler on the information superhighway, it is one’s own responsibility educate themselves on how their technology works and how to navigate around the web safely, as well as to stay informed of changes in the way things work. This is the same as the way a driver on the actual highways has to educate themselves how to operate their vehicle and to navigate the roadways safely, and to stay informed on changes in the laws and if your vehicle changes.