The wp_login action hook is triggered when a user logs in by the wp_signon() function. It is the very last action taken in the function, immediately following the wp_set_auth_cookie() call.
This hook is an action which means that it primarily acts as an event trigger, instead of a content filter. This is a semantic difference, but it will help you to remember what this hook does if you use it like this:
<?php
function your_function() {
// your code
}
add_action('wp_login', 'your_function');
?>
This hook provides access to two parameters: $user->user_login (string) and $user ( WP_User ). To pass them into your function you will need to add a priority (default is 10) and request 2 arguments from the add_action() call:
<?php
function your_function( $user_login, $user ) {
// your code
}
add_action('wp_login', 'your_function', 10, 2);
?>
See Plugin API - Actions for more detail.
wp_login action hook is located in wp-includes/user.php