They bring sites back to their home database. You type in a keyword and the search engine matches your word with the words in its database. Think of the millions of words involved. No wonder you tend to come up with millions of sites, most of which are not helpful to you. The trick is writing a good search.
Many search engines are case sensitive.
Think of the words you would like to see in the site you find. Put the most important ones first. If the word must be in the article, put a plus sign before the word. If you don't want a word, put a minus sign before it. Search engines AltaVista A good search engine.