Collection and organization is done by humans and computers. They are best used for browsing more generalized topics and will guide the user in an efficient manner. Many search engines now offer directories on their sites for browsing their databases.
Virtual libraries can vary in their reviewer's abilities to properly evaluate sites.
The human factor also limits the number of sites that can be included. Help Indexes web pages, Usenet Newsgroups. Indexes the entire web page. Searches other search engines simultaneously. Uses 13 other search tools to look for news, weather, stock quotations, etc.