Gray hat SEO is SEO techniques that take more risks than white hat SEO techniques but aren't likely to get your site banned from search engines (although a search engine penality could result). They're questionable SEO techniques but not in the same category as black hat SEO techniques. However, what's considered gray hat SEO today might be black hat SEO next year.
Black hat SEO is a practice that increases a page's rank in search engines through means that violate the search engines' terms of service. Black hat SEO is a type of unscrupulous search engine optimization tactic used to raise a website's search engine results page (SERP) ranking illegitimately or lower the ranking of a competitor's site.