They usually have actual people that will sort through the listing of web sites that come in and decide what category the site should go under. They will have Category listings to guide you to sub-categories until you break it down to a specific enough topic to return web site addresses. Portals also have some of the disadvantages in that they control the content that you are able to find. They are a good aid for new users to get acquainted with what's out there and what can be done, but at the same time, they only give you a small part of the Internet. Portal sites often point you to sites that are affiliated with their site, rather than to the most relevant or informative site on your searched topic.
Some search engines are meta search engines.
These engines do not maintain their own databases but search those of other engines. Of course, this means that MetaCrawler is slightly slower than other engines, but is more likely to obtain accurate results for your query. Dogpile is a relatively new meta search engine which is also a popular tool. The last type of search engine is called a micro search engine because it specializes in a category. There are micro engines for all different kinds of occupations and hobbies.