Google’s webmaster guidelines are a set of advices and recommendations edicted by Google to appears in its SERPs. From the Google’s point of view webmaster guidelines are a way to ensure a good indexing of website pages and a good user’s experience via pertinent results.
Google differentiates three types of guidelines:
Design and content guidelines
Quality Guidelines outline illicit practices that may lead sanctions - index removing or bad rankings.
Some examples of "illicit" techniques:
An excerpt of "Design and content guidelines":
The full guidelines.
SEO practices against Google’s guidelines are usually called black hat SEO.