Alerts are automated notifications that can keep you up to date with the literature in a given field.  Many databases send users an email or updated RSS feed when a new book or article comes out that matches any search criteria the user chooses.  Additionally, users can receive an alert every time a new book or article cites a particular source.

Individual databases may refer to these services as "alerts" or "saved searches."  Some may require the creation of a user profile.  xSearch and Google Scholar are commonly used to create alerts that span multiple databases.