Types of search engines

There are different types of search engines.

1) Crawler / Spider or robot based search engines - Search engines like google and yahoo has their own spider or crawler software which automatically visits and collects data of different websites and this data gets included and indexed in search engine database . Each search engine has its own algorithm to have ranking of these websites and display the search results.

2) Search Directories - has categories and subcategories for listing the websites. One can submit his website to suitable category and then it is reviewed by human editors and then they decide the listing and ranking of the website. e.g. dmoz.org , dir.yahoo.com

3) Meta search engines - This type of search engines searches the major search engines like google, yahoo , ask.com , bing etc. simulatnously at a time and displays the combined result on search engine result page (SERP) . e.g. dogpile.com

4) Specialized search engines - These are search engines for specific niche area.
e.g. to search books we have books.google.com
To search blogs we have blogsearch.google.com
To search articles we have scholar.google.com