SharePoint has a built-in alert mechanism that enables users to register for different kinds of alerts. Basically, a user selects the piece of content he wants to be alerted on and requests that SharePoint send the requested user an e-mail when that content changes.
For example, you might use alerts with a document library that is supposed to have documents regarding a specific topic. You might want to know immediately when a new document is added to that document library. SharePoint can send you an e-mail each time a new document is added.
To learn how to set up alerts regarding all content within a specific document library, or to set up alerts relating to a specific document, review the Overview - How to Set Up Alerts guide and follow the step-by-step instructions.