Strictly speaking, email address harvesting is the process of collecting viewable email addresses on the Internet.
The automated harvest is done by a spider or bot on webpages, forums, blogs, etc. Email addresses are easily recognized as a character string with an @ and an extension.
Some harvesting softwares or bots are able to partially qualify the addresses by analysing the context and by using SERP related to a specific topic as a starting point.
See also email extractor.