A search engine is a set of programs which are used to search for information within a specific realm and collate that information in a database. People often use this term in reference to an Internet search engine, a search engine which is specifically designed to search the Internet, but search engines can also be devised for offline content, such as a library catalog, the contents of a personal hard drive, or a catalog of museum collections. Search engines help people to organize and display information in a way which makes it readily accessible.

There are three aspects to a search engine: crawling, indexing, and searching. When a search engine crawls, it looks for new content which was not present during the last crawl, including updates to files and web pages. Then, the programs index the information, pulling out specific keywords to categorize it. On the Internet, for example, indexing relies heavily on keywords in web pages, and meta tags which provide information about the page.

Once the information has been indexed, information about how to access it is stored in a database. Some search engines also store or “cache” information to make it easier to retrieve. When someone searches the database, it spits out results ranked by relevance. On wiseGEEK, for example, a search for “giraffe” will turn up articles related to giraffes, including, of course, “What is a Giraffe?,” for the convenience of the user.

While all search engines work in similar ways, their usefulness can be quite varied. Search engines rely heavily on complex algorithms to rank the relevance of their search results, especially search results for common keywords. Users tend to gravitate to search engines which return results they like, with Internet sites like Google®, Yahoo!®, Lycos®, and AltaVista® warring for users with various features which are designed to make their searches more appealing and more relevant.

Google® has managed to become so associated with the process of conducting a web search that web searching is colloquially known as “googling,” as in “let me google that and see what comes up.” The Google® company is actually not very pleased with this, as the company fears that lower case use of its name, along with generic use, could contribute to trademark dilution.

Many Internet search engines are smart enough to learn from their users, incorporating user activities into their relevancy rankings. Search engines also rely on information like links from other pages and a site's reputation to rank search results, all within the fraction of a section. Adept users can sometimes manipulate search results in a practice known as Googlebombing, in another reference to Google®'s search products.