Search engines use software robots to survey the Web and build their databases. Web documents are retrieved and indexed. When you enter a query at a search engine website, your input is checked against the search engine's keyword indices. The best matches are then returned to you as hits. Keyword searching This is the most common form of text search on the Web.
Most search engines do their text query and retrieval using keywords.
Essentially, this means that search engines pull out and index words that are believed to be significant. Words that are mentioned towards the top of a document and words that are repeated several times throughout the document are more likely to be deemed important. Most search engines index every word on every page. Others index only part of the document, such as the title, headings, subheadings, hyperlinks to other sites, and the first 20 lines of text. This often results in hits that are completely irrelevant to your query.