Search engines employ software robots to survey the Web & build their databases. Web documents retrieved & indexed through keywords. While you enter a query at search engine website, your input is verified against the search engine's keyword indices. Then the best matches are returned to you as hits. For verifying, it uses any of the Search algorithms.
Search Engines employ software programs known as spiders, robots or crawlers. A robot is a piece of software which automatically follows hyperlinks from one document to the next around the Web. While a robot discovers a new site, this sends information back to its main site to be indexed. Since Web documents are one of the least static forms of publishing (that means they modify a lot), robots also update earlier catalogued sites. How quickly & comprehensively they carry out these tasks vary from one search engine to the next.