SE is a program that searches documents for specified keywords and returns a list of the documents where the keywords were found.
Search engines use automated software programs knows as spiders or bots to survey the Web and build their databases. Web documents are retrieved by these programs and analyzed. Data collected from each web page are then added to the search engine index. When you enter a query at a search engine site, your input is checked against the search engine's index of all the web pages it has analyzed. The best urls are then returned to you as hits, ranked in order with the best results at the top.
It can be considered as a program that searches for a specific word or groups of words throughout the web and creates a list of the Web pages that contain the specified word(s) Typically, a search engine works by sending out spiders to fetch as many documents as possible for given keywords.