The Trigger module provides the ability to trigger actions upon system events, such as when new content is added or when a user logs in.
The combination of actions and triggers can perform many useful tasks, such as e-mailing an administrator if a user account is deleted, or automatically unpublishing comments that contain certain words. By default, there are five "contexts" of events (Comments, Content, Cron, Taxonomy, and Users), but more may be added by additional modules.
For more information, see the online handbook entry for Trigger module.