Black Hat SEO is also known as SEO spamming, and is when someone creates websites that have little or no value, that are intended to trick the search engines into offering irrelevant or low-quality search results.
Black Hat SEO will heavily leverage technology and automation in order to create tons of poor quality sites, and gain inbound links from other websites (link spam). Once the visitor has arrived to the website, the owner of the website will often use some kind of cloaking or redirect to show the visitor a sales page of the products, that is the really goal of the Black Hat SEO technique.
Black Hat SEO are all about getting fast results. The method they use to optimize their websites will often get the website to the top of the search engines very rapidly, but when the search engines catch-on, the website will very often crash or get banned.
I do not condone Black Hat SEO for affiliates, because it is very risky SEO business, and it can especially damaging, when it is used on a website, that you have invested heavily resources in.