Content spamming, in its simplest form, is the taking of content from other sites that rank well on the search engines, and then either using it as-it-is or using a utility software like Articlebot to scramble the content to the point that it can’t be detected with plagiarism software. In either case, your good, search-engine-friendly content is stolen and used, often as part of a doorway page, to draw the attention of the search engines away from you.
Everyone has seen examples of this: the page that looks promising but contains lists of terms (like term term paper term papers term limits) that link to other similar lists, each carrying Google advertising. Or the site that contains nothing but content licensed from Wikipedia. Or the site that plays well in a search but contains nothing more than SEO gibberish, often ripped off from the site of an expert and minced into word slaw.
These sites are created en masse to provide a fertile ground to draw eyeballs. It seems a waste of time when you receive a penny a view for even the best-paying ads but when you put up five hundred sites at a time, and you’ve figured out how to get all of them to show up...