Websites even when they are common with one another have to be given categories and leveling in order to make clear distinction and classification. The purpose of classification is, of course, because of the need to for an easy understanding about what they are for.  Web 2.0 websites embody this principle of hierarchy and leveling.  Below are the four levels of websites for the Web 2.0

a. The Level 0 are the websites that are capable of being worked on both online and offline mode.  Some of the more classified example that belong to this category and level are the Mapquest site, the Yahoo!Local and the very famous and widely used in the world the Google mapping.  These websites are practically capable of being worked online and offline. 
b. The Level 1 refers to the application websites that are capable of being worked and operated offline however, has gained online features.  This means hat you will need to be directly connected to the Internet when you would like to make use of this feature. 
c. The Level 2 refers to the applications that are capable of being operated and used offline but will essentially be strong and powerful when used and operated online.  The Flickr website is one of the classic examples that belong to this level of web 2.0 web applications.
d. The Level 3 are those applications that are basically web 2.0 based.  They are the websites that practically work and exist when there is the Internet.  Some of the more noted examples that belong to this category are Wikipedia, Craiglist, Google Adsense.