Search engines are programs that index and search information on the web. Since it is not possible to search the entire Internet with a single search, in order to be comprehensive, you'll need to use several different types of search engines.
Types of Search Engines
General Search engines compile huge databases of web page files with robots or spiders. 
Typically, these spiders or robots will index most of the words on publicly available webpages. Whenever you search the web with a search engine, you're asking the engine to scan its index of websites and match your keywords/phrases to those found within website document texts. Search engines also rank results--some will do it based upon the number of times your keywords appear in the document, and how close the words appear together. Others rank by popularity, that is, by the number of other pages that link to a page. Google is one of the search engines that ranks by popularity.
Invisible Web or Deep Web search engines. A huge portion of the web, currently estimated to be 400 to 550 times larger than the commonly defined World Wide Web, is "invisible"; that is, the information within these "invisible" websites cannot be located by spiders or robots. Invisible Web or Deep Web information is behind a wall, or rather, within a database. To access Invisible Web information, you'll need to first locate the database.
Metasearches search multiple engines at once and typically display results within a single result list. Dogpile is a good example of a metasearch engine.
Specialty search engines index and search particular item types such as news articles, blogs, books, or discussion groups. They may also focus on a particular subject area. Popular specialty search engines include: Google Books or Google Scholar.
Natural Language search engines use Natural Language Processing (NLP) to linguistically analyze your question and act as question answering system. In these types of search engines, searches are phrased such as "How much has the price of gas increased?" Ask and Bing use natural language search engines.
Image source: Google. Wikimedia Commons.
Google Scholar provides an easy way to search for scholarly literature across many disciplines and resources.