Black hat refers to the use of underhanded, sneaky, parasitic or malicious tactics.  It’s a general term that applies to hacking and security, but in our case we’ll limit the definition to black hat search engine optimization.

Black hat SEO is the use of of any tools or techniques specifically designed to manipulate search engine results through misleading or extremely low quality (spammy) content.

Black hat techniques include, but are not limited to:
Cloaking
Forum spam
Blog comment spam
Insertion of hidden links in sites by exploiting security holes
Unattributed paid links

The definition is vague enough that some people will include any sort of automation, content spinning, and link networks.