This block shows the login-form for only anonymous users who still are not passing the authentication. If the user registration is allowed, it shows the link to register.php. And, if the SSL login is allowed, it shows the link specified by options of preferences.
You can do login in user.php without this block, if you has uninstalled this block by mistake.
This block shows a name of users who may be accessing to your site and numbers of them.
Getting correct informations for online users is too heavy process. For that, this block doesn't try to find the correct results. But, it's enough to make your site exciting.
This block shows a name of users who are registered to your site recently.
This block shows the ranking of the top posters.