In search engine optimization (SEO) terminology, Black Hat SEO refers to the use of aggressive SEO strategies, techniques and tactics that focus only on search engines and not a human audience, and usually does not obey search engines rules. Some examples of Black Hat SEO techniques include keyword stuffing, invisible text and doorway pages. Black Hat SEO is more frequently used by those who are looking for a quick financial return on their Web site, rather than a long-term investment on their Web site. Black Hat SEO can possibly result in your Web site being banned from a search engine, however since the focus is usually on quick high return business models, most experts who use Black Hat SEO tactics consider being banned from search engines a somewhat irrelevant risk. Black Hat SEO may also be referred to as Unethical SEO or just spamdexing, as spamdexing is a typical frequently used Black Hat SEO practice.
I must be one amongst millions who receive tons of emails offering 1000 backlinks for a few dollars - these must surely be black hat links ? How could they be otherwise
A good description, particularly the part about the search engines rather than human audience. Essentially it is a way of using some very clever but less than ethical techniques to get to the top of a search engine. Unsolicited mail shots are often defined as black hat, and it's used in other marketing areas to talk about unethical methods. As it's been said said it can be very lucrative, but it has to be reinvented every few weeks or months as the search engines catch up and you will be in an arms race with Google.
BlackHat is something that is going to get you hurt in the long run. A lot of individuals who use those techniques are doing so because they are not looking at a website from a long term stance. This can be beneficial to them because they are looking to get their money and run in a sense, but it doesn't mean that Google won't follow you around once you end up putting getting branded. I've heard too many domainers/webmasters end up getting banned from various affiliate programs. Its very use at your own risk.