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White hat SEO is to make a good quality site, and then to let the rankings come naturally as a result of your quality. Relying heavily on the advantages of quality content and effective marketing strategies to achieve top search engine rankings, sites that follow White Hat SEO write content for people and not search engines.

Black Hat SEO occurs when someone creates websites that are targeted at tricking and manipulating search engines and thereby get irrelevant or poor quality search engine result pages (SERP). It is doing everything that the search engines say you shouldn't, the techniques includes cloaking, automated link spamming, anything hidden (including links or text), etc

Gray hat SEO are questionable SEO techniques but not in the same category as black hat SEO techniques. Gray hat SEO includes things like publishing duplicate content, mild artificial linking schemes etc.