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.