WordPress at a glance
action is not described

wp_login action-hook . WP 1.5.0

Fires after the user has successfully logged in.

Usage

add_action( 'wp_login', 'action_function_name_6205', 10, 2 );
function action_function_name_6205( $user_login, $user ){
	// action...
}
$user_login(string)
Username.
$user(WP_User)
WP_User object of the logged-in user.

Where the hook is called

wp_signon()
wp_login
wp-includes/user.php 109
do_action( 'wp_login', $user->user_login, $user );

Where the hook is used (in WP core)

Does not used.