Search engines are electronic billboards.
Well to attract attention, they allow people to enter in some words that they are interested in and will spit out a list of web pages containing those words. So for instance, if you were interested in me, you could type in my last name to see if there are any web pages about me. Then while reading an add for toothpaste, you would see a list of all the known web page that have my name on them. By the way, you will also find stuff about horse racing. Each search engine does this differently.