Last night in my Internet Searching and Web Design class I learned that back in the real heyday of the internet in the 1990's, spammers figured out a few tricks to get their webpages up to the top of the ranking.
Wikipedia has a great overview of Spamdexing, tricks spammers have tried to manipulate web search engines.
One in particular involved loading their pages with certain keywords because the frequency of an occurrence of a word in a page would grant that site relevancy.
Luckily search engines are more sophisticated now but what cracks me up is that spammers would put all these words in their pages but then hide them by coloring the text the same as the background. So back in the day when you searched for info on say "autos" and got a XXX site that's 'cause they secretly typed out 'autos autos autos autos' all over their webpage! heh. (which frankly you might not have even noticed even if the text WAS visible on an XXX site....)
Anyway, now engines are a lot smarter and can do things like search for color on color as well as check for too MUCH frequency of a word.