Search engines access databases constructed ``and updated by programs called 'robots' or 'spiders' that continually 'travel' the Net finding documents. As they encounter documents, they record information about each document, which is then used to update the database of documents." [2, p. 70].
The databases are built over time, as the ``robots" and ``spider" travel the Internet. Some date the entries and ``drop them" after a period of time (not all do!). As a result some of the information gathered may be out of date.
But this is not a reason not to use theses services.
The complexity of the search engine, and of the interface, varies from tool to tool. But the output from the searches, in all cases, results in a list of URLs that match the search string.
There are a number of good search engines. Some of them include