A citation alert notifies you by email whenever a publication (e.g. journal article) you have chosen has been cited by a new publication that has been added to the database.
Although different databases will have different procedures for setting search alerts, the general principles are same. You must:
Google alerts are a great tool for research, as you can set an alert so that Google searches the web on a regular topic gathering hits on a topic of interest from a range of sources (including news sources, videos and blogs).
*You can run a number of alerts concurrently, and revisit your Google Alerts account to modify/add/remove alerts.
For advanced Google searching tips, look towards the Advanced Google searching guide.