Most information on the Web does not go through a review process.
Anyone can publish on the Web without passing the content through an editor. Pages might be written by an expert on the topic, a journalist, a disgruntled consumer or a sixth grader.

Some information on the Web is not free.
Many Web pages are free to view (and actually many of the best ones are), but some commercial sites will charge a fee to access all or part of their information.

Information on the Web is not organized.
Some directory services, like Google and Yahoo, collect links to sites and place them in subject lists. But there are too many Web pages for any single directory service or search engine to organize and index.

Most information on the Web is not comprehensive.
The millions of Web pages out there make up an eclectic hodgepodge of information and opinion. Rarely will you be able to use a search engine on the Web to collect information about your topic from different decades, different viewpoints, and different types of sources.

Most information on the Web is not permanent.
Some well-maintained sites are updated with very current information, but other sites may become quickly dated or disappear altogether without much notice.