Spam is a term used to describe unwanted or malicious content that is user generated. It appears on a publisher’s website as a result of a third party posting a comment, subscribing with some sort of message field, filling out a form and other ways. These messages usually contain a link back to the submitter website giving them what they think is a desirable “back-link” but in truth that type of spam not only hurts the sites publisher but is ignored or penalized by search engines. Despite this Spam is very common and undesirable.
Too much allowed spam may result in the website appearing to search engines as a kind of link farm which will penalize the sites ranking. Importantly, viewers of the website will take the site less seriously if they see comments full of spam because it signals the user that the site owner either does not care if there’s too much abusive commentary or they simply do not know to control it (even worse from a reputation point of view).
Spam can be controlled in two ways. Manually reviewing all comments before publishing them or using software (plugins) to eliminate the majority of spam. One such program comes with WP themes called Akismet. Akismet keeps a data base of common spammers and uses its built in mechanism to identify sand isolate spam type comments. This is very helpful and advice. There are third party spam detectors as well. You must activate Akismet in the WordPress admin area. It is free for smaller websites and is paid for by larger commercial websites.