Search engines are computer programs that can be used to browse documents or databases. These programs usually work on the basis of an index, which is created either manually or by use of crawlers. The search results are output, based on algorithms which determine the relevance of a document with respect to the search query. Today, the term “search engine” is almost exclusively used for web searches. The Google Search is one of the world’s leading search engines.
Electronic documents are very common since the Internet breakthrough. In addition to many millions of websites, digital archives exist in libraries. Web shops such as Amazon house millions of articles which have to be found by respective users. In order to output the digital data in a structured format, search engines were created. Search engines have long since replaced popular web directories such as DMOZ especially for navigating through the Internet.
For a search engine to be useful at all, it needs an index. This contains a structured collection of data, which can be output for corresponding search queries. Depending on the type of search engine, the data is obtained by a crawler, fed in or read in. To answer a search query, the search engine must be able to provide relevant results to the query. The more elaborate the system, the more user-friendly the search results will be.
A search engine is defined on the basis of different characteristics, which in turn are determined by the search engine type.
Search engines are initially characterized by a search bar into which the corresponding query can be entered. When outputting the results, a small portion of the document is displayed, called a “snippet.” In the web search, both the page title and the meta description are used for the snippet, if available. Other search engines, such as an intranet search of a library, show the titles of the books or documents matching the search query.
Depending on what is intended with the query, different web search engines can be used.
For example, there are:
These special search engines are part of a vertical search. Providers such as Google or Microsoft combine several different vertical search functions in their respective search engines.
Most search engines are index-based, in other words, the program first searches documents and websites and then creates its own index. In the case of a search query, the program finally accesses this index and outputs the results by relevance.
A search engine must first interpret the query to compile the relevant results from its index. For this purpose, the request is adapted in such a way that the internally working algorithm can understand it. Modern search engines can also provide answers to complex questions or sentences. This is often called a semantic search engine.
There are other factors in addition to the algorithms which increase the relevance of the search results to the search term. Thus, search engines may also evaluate user agents as well as location information if, for example, a mobile search is performed. Moreover, search results can differ depending on whether the search history has been saved in the browser or if you are logged in.
An important part of the search process is the evaluation of the available data. In web search engines, this is called ranking factors. For example, the link structure of a website and its age influence the ranking with regard to a specific search term. Current web search engines provide only a few hints on how pages are classified to avoid webspam.
Search engine are used for different purposes, information retrieval, for example. Users can be further classified according to their search intent:
As of February 2014, Google is the leading search engine in Germany, with a market share of over 90 percent.
In the US, Google is also ahead (as of January 2014), with a market share of 67.7 percent, followed by Microsoft with 18.3 
In other markets, such as Russia, the Baidu search engine of Yandex in China is the biggest provider.