If you wanna know the unethical, dark side to SEO look into Black Hat SEO. It involves doing things that google and search engines don’t recommend. Things like: Invisible text, Link spamming, Keyword overuse, Page changing and Keyword irrelevancy.
If you have ever come across a website which is stacked with ads and really only gives you bland content with spelling mistake and a lot of focus on linking to other irrelevant content, good chance Black hat SEO was used here. Black Hat SEO is deployed to make a quick buck, get mass views and referrals. It’s also a good thing that Google is actively banning and penalizing those who do Black hat SEO, this reason is a god sent to those who take the time to produce good content for natural SEO ranking.
Although sometimes Google may penalize you accidentally for what it sees as Black Hat SEO or search engine manipulation such as keyword spamming. Mostly it’s not an issue, Black hat SEO is mostly purchased and done en mass so stick to popular SEO guides and methodology and your site will rank fine…if you put the time in and have quality content.