If you are coming to this page for the first time, you'll see form into which you can enter your username and some other settings (crypted password and e-mail are stored). If you click on Create Profile, a user profile will be created for you. With the response, a HTTP cookie will be sent that contains your user ID, which enables the system to recognize you.
Logout clears the cookie. As described above, Login enables you to gain access to your user profile if you somehow lost the cookie or are on another machine. Save just updates your profile.