If you pay close attention to your Google Analytics (GA) data, you might see page links that does not belong to your site at all. This is what Google Analytics spam does. GA spammers inject themselves into Google's analytics servers in order to raise the SEO and page rankings of their spam websites. Don't let them cloud your data by setting up your views and filters correctly. These slides are from my WDN presentation.
Slides:Preventing GA Spam using Views and Filters