The term Blackhat is used to describe different methods used by SEOs to manipulate the placement of sites in the SERPs.
Its mainly used to describe techniques, which are considered unethical. However, even submitting a link for ones website, with the intention of increasing pagerank, or otherwise manipulate the SERPs, would be a manipulative behavior, and as such qualify as a blackhat technique.
Spamming is by far the most common technique, usually its been automated, because placing links manually is a tedious task.
Linkbuilding is usually incorrectly described as a whitehat technique, but as it is done to manipulate the SERPs, this belongs to other blackhat techniques, regardless if its done manually or not.
Including long lists of keywords in the content, (both visible and hidden). Stuffing the content unnecessarily with keywords, in a way which makes it unnatural to read.
Doorway pages are fake pages, used to give search engines the wrong picture, and make a page rank for keywords not even relevant to the page. They are only shown to the search engines, while users get a normal page.
Doorway pages are created by reckless and irresponsible SEOs, with no life, aside from that they got from spamming and deceiving people into clicking their useless junk sites.