LogIn and Registrations (category)

authenticate	Allows you to perform additional checks on a user's authentication credentials (login/password) before the user is authenticated.
login_footer	Fires after the form is output on the login/authorization page (wp-login.php).
login_head	Fires in the `<head>` section of the login/registration page (`wp-login.php`).
login_header	Fires immediately after the opening `<body>` tag on the login/authorization page (`wp-login.php`), before the form is output.
login_headerurl	Allows you to change the URL of the logo link above the registration/login form. By default, the link points to wordpress.org.
login_redirect	Allows you to change the URL to which a user is redirected after logging in.
login_url	Allows you to change the login page URL (link).
register_form	Fires after the “Email” field is displayed in the user registration form on the `wp-login.php` page.
registration_errors	Allows you to add custom registration checks and change error messages on the registration page.