The powerhouse of search. In most countries, Google runs a solid monopoly on the search market. As always, that kind of massive influence brings great responsibility.
Google has been taking this responsibility very seriously. The Google team is dedicated to giving searchers the highest quality and most relevant results - evidenced by their many algorithm changes meant to weed out shady and low-quality websites.
It's an ongoing process, and standards of quality are evolving with tech and social values.
There are a lot of ways to end up on Google's shit list: shady link building, keyword stuffing, overloading your site with anchor text, etc., etc. Some of these things will get you a slap on the wrist, others will get your site completely banned from Google.
Some advice from the experts: you can't go wrong with valuable, high-quality content. No matter what else Google changes, great content will see you through.
Write your content for people, not search engines. Google will only get smarter, and algorithm changes will continue to crack down on black hat SEO.
Here's a history of major algorithm changes that changed the way we optimize our websites.
Infographic courtesy of HubSpot.