admin_notices
admin_notices is an action fired in the WordPress admin, under the submenu, but before the page's content starts rendering. It is intended to be used for plugins users to pass messages to users. Check out some of WordPress' own notices and try to emulate its HTML and CSS for a consistent user interface.
This hook provides no parameters. You use this hook by having your function echo output to the browser, or by having it perform background tasks. Your functions shouldn't return, and shouldn't take any parameters
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: add_action('admin_notices', 'your_function');
This hook was introduced in WordPress 2.0.2, and will not work in earlier versions.