Google Alerts are a great way of keeping in touch with certain changes in the Google index. This can be used to track keywords, news, a favorite device or celebrity and anything else that is indexed by Google. The user can define a few parameters including the interval in which the alerts are send out and the type.
The default type is comprehensive which means that Google observes everything for the user. It can be changed to video, blog or web related alerts which would exclude the rest of the types. Alerts can be send out once per day which is the default value. That can be changed to once per week and to when it happens which could mean that you get bombarded with emails if you happen to observe a popular term.