Black Hat SEO is an unethical technique done to gain higher ranking. The purpose of SEO is to get traffic & ranking and it should be in proper way and also has some limits. If u do this in improper way then it is called black hat or SEO spammer. It includes link farms & keyword stuffing etc.
Spamdexing (also known as search spam, search engine spam or web spam) involves a number of methods, such as repeating unrelated phrases, to manipulate the relevancy or prominence of resources indexed by a search engine, in a manner inconsistent with the purpose of the indexing system
jeffersondaniel explained it quite well, but I'd like to add that Black Hat is any technique specially designed to artificially increase the ranking of a website or mislead the search engines into thinking that your have more content of authority that what you really have. I recommend that if you want to perform some Black Hat SEO, then a) don't do it on a site that you value in particular and b) make sure that you valuable sites don't share the same IP with that particular site.
Black Hat SEO is a technique which is used to get more traffic and hirgher ranking in Search Engines in unethical way. Black Hat SEO techniques generally don't follow Search Engine's rules and present irrelevant content to user and search engines. White text, Keyword stuffing, doorway pages, interlinking are some techniques of Black Hat. But I strongly suggest you not to do it. Black Hat can give you good result at start but when it will caught, your site can get penalize or can be blacklisted.
Search engines may punish sites they discover using black hat methods, either by reducing their rankings or eliminating their listings from their databases altogether.
It is an illegal way of site promotion and has a possibility to block that site by google...link farming, hidden texts, etc are some black hat SEO
Blackhat seo is unethical way to promote your site eg. hidden text, doorway pages, cloaking, keyword stuffing, link farm..
The problem with Black Hat is that to be good at it you really need to be very netsmart and illusive. If you're clever enough to do it and not get caught, you're clever enough to market ethically ('whitehat'), make more money long term, and not spend your online career looking over your shoulder. Unless you want a life of second-guessing and evading Google (and some of the biggest computer brains in the world) then forget it! Alun from England
blackhat seo in simpler words means doing search engine optimization in an illegal way. which is against the google rules..
In my experience, I did some experiement comparing both, using black-hat can get you to rank 1st 10x faster on keywords. Using it is a personal choice, and you can use black-hat tools with a white hat approach.(EVERYONE exept you will think it is white-hat) Black hat isn't all negative, its rather misunderstood, using it can get you problems and be a pain for everyone else. Like spamming for viagra and such.
Funny thing is that some of it comes down to intent. Sure there are some things that are plain and simple whit or black, but... I'm just sayin' : )
Anything that's against the TOS of the searchengine you're using. And in the case of Google a big one is trying to manipulate the rankings. Optimizing and manipulating are two different things. In a sense intentially "building backlinks" to improve rankings falls under BH.