Black hat SEO refers to search engine optimization techniques that are against search engine guidelines - especially Google’s guidelines - or considered as unethical or even sometimes illegal. More often the goal of black hat SEO techniques is to trick Google’s algorithms.
Black hat SEO is also sometimes defined as deceptive practices but this definition is questionable as black hat techniques may be used for good or pertinent content and therefore not be really deceptive.
Black hat SEO is often used to give good SERP ranking to low quality websites but can also sometimes be used to foster ranking for "legitimate" or well established websites.
Black hat SEO is risky because when discovered, the website can be penalized.
The distinction between black hat and white hat SEO is not always easy and in this case the term grey hat SEO is sometimes used. Strictly speaking, over-optimization is normally different from black hat SEO.
As for classic SEO, there are on-page and off-page black hat SEO techniques.
More known on-page black hat SEO techniques are:
invisible or hidden text
Agent delivery cloaking
Off-page black hat SEO techniques are dedicated to gain many artificial or inorganic backlinks. They are often used because they are not easily discovered and sanctioned when used intelligently. These techniques are especially addressed by Google’s Penguin.