Staying up-to-date and using content to your advantage is one of SharePoint's usage scenarios. Alerts are a great way to select content where you want to be updated. This way you can work with fresh and updated content to maker smarter decisions. You can get in contact with people who have the same interests.
Content in SharePoint where you can set an alert includes list items, documents, lists and document libraries.
Before a user can use alerts a SharePoint Farm Administrator needs to enable them inside Central Administration under Application Management.
How to create an alert on a document in SharePoint
Go to a document, open the drop down menu and select the alert me item to set a SharePoint alert directly on a document. If you want to remove the alert you can access your alerts using the my settings page.
You can choose when you are notified of changes e.g. if a certain condition is met like a new document was added or an existing document was modified. You can also set a time frame when you are notified like daily or weekly.
Handling Alerts for other People
You can manage alerts for other users if you have the permission to manage site settings. Going to Site Settings > Site Administration > User alerts you can delete an alert for a different user.