They are companies that run complex programs to keep track of all the sites on the Internet. Each search engine has something to offer. Altavista, for example, indexes everything about a site, while Yahoo is a subject index. Some search engines let you use key words or phrases. Most offer directories for general categories.
Most search engines are free.
They sell space to advertisers. Here is a sample of advertising. Click - Press your left mouse button. Double Click - Press your left mouse button twice in rapid succession. Download - To transfer a computer file from a remote location to your own.