Grey hat seo is tech. which comes in between white and black hat seo .Grey hat has some techniques of white hat but it also do unethical seo like spamming but less than black hat seo.So Grey hat seo works on both ways ethical and unethical seo.
Grey hat seo is combination of Black hat and White hat seo. It includes both activities to promote a website or blog. These days most of the webmasters are using this new seo technique. However, there is also chances of ban if we cross limit.
I think the strategy in gray hat seo is making backlinks more than white hat seo but smaller than black hat seo too. I myself only suggest white hat seo which is recommended by Google and other search engines and is totally safe.