Tag Spam Definition
This type of spam is almost useless nowadays but when combined with other types of spam it has some effects which makes it a top priority on our list of spams to hunt.
Wikipedia has a great article about tags and tag spam entitled Tag (Meta Data)
Tagging systems open to the public are also open to tag spam, in which people apply an excessive number of tags or unrelated tags to an item (such as a YouTube video) in order to attract viewers. This abuse can be mitigated using human or statistical identification of spam items. The number of tags allowed may also be limited to reduce spam.