A Spammer is a type of troll that spams forums with advertisments or other content. Spammers are mostly automated and used by website or product owners to advertise there website and/or product. This type of Spammer is often called a "Spam Bot". Sometimes, however, Spammers can be human and sometimes create junk threads on forums or simply post annoying (Not trollish) content.
Spammers are best ignored, however if in large amounts they may become quite dangerous, as demonstrated in the Paleo Insanity Incident.