have you heard of black hat? It's "underhand" SEO/marketing - eg back in the day people would have stuffed their (very basic) html pages with keywords which were the same colour as the bacground or used 1px images with keyword stuffed alts and anchor text. Spam of sorts; using bots to auto post on forms etc to build incoming links Pushing your product or page to people who probably are not intested, lots of popups... white hat is following best practice : being good & following guidelines grey hat is somewhere inbetween....edging on blackhat....perhaps a little sneaky
Whitehat = doing exactly what Google says you should do (follow their webmaster guidelines). Blackhat = doing stuff that Google don't want and if you get caught you'll face their wrath. Greyhat = the normal SEO stuff that everyone does anyway and won't get you banned, but Google isn't happy about.
I would say greyhat SEO is anything that is somehow shady but you still get away with it (no google penalty).
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