Grey hat would be questionable, such as creating blogs for the sole purpose of backlinks, or maybe link exchanges. It's an opinion. The search engines (Google) states that buying links, javascript tricks, keyword stuffing, etc. The thing is that Google's Matt Cutts has said some things that some would consider grey hat, or even black hat. see - http://searchengineland.com/think-like-a-blackhat-by-matt-cutts-16129 Basically, if in doubt, don't do it. Or, do it, but accept the risk.
Black hat tactics are those which the search engines do not consider ethical. It is much spammy method. Grey hat means less spammy than black hat technique.
Grey hat can be defined as a risky SEO technique. It might work well for you and you also might stand a chance of getting penalised... Grey hat techniques are a combination of white hat and black hat techniques. Having said that, we can also say that it is just taking advantage of some ambiguity in search engine algorithms.