This is important.
There are good and bad ways to optimize your site. The bad ways are called 'Black Hat'. Sure, they may work for awhile, and some Google can't (or doesn't bother to) pick up automatically. However you can report a site to Google as using Black Hat SEO tactics and Google will remove that site from the index (meaning it won't show up in search results). Removing a site from the index is usually only done for a certain amount of time and can be appealable. Banning is far more severe and banned sites are often gone for good with no way to get Google to add it back to their index. Beware of a lot of things that seem shady. If you think they are shady, chances are that the folks at Google will think so too and if one of those other sites in the SERPs wants to rise up and they visit your site and see your shady tactics, they won't hesitate to report you