you choose a topic/theme/keyword that you are interested in and then everyday GG will send you an email related to that theme, including new website, new post, etc.
Google Alerts are email updates of the latest relevant Google results (web, news, etc.) based on your choice of query or topic.
you have to type your query and if google index anything related to your search query then google will mail you
You can choose any term - E.g Pagerank and google will email you news/results surrounding the term you specified.
Google Alerts will monitor certain keywords that we requested, then google will index all existing web pages and reported to us after finding a page that identified