Tags can have a type. Currently there are only five types: artist, character, copyright, species, and general.
Artist tags identify the tag as the artist. This doesn't mean the artist of the original copyrighted artwork (for example, you wouldn't use the ken_sugimori tag on a picture of Pikachu drawn by mellis).
When tagging something, you can tell 20% Cooler that a tag is an artist tag by prefixing it with artist:. For example, tagging something artist:mark tree will tag a post with mark and tree. If the mark tag doesn't already exist, it'll be created with the tag type set to artist.
Character tags identify the tag as a character. They work exactly like artist tags, only you prefix with "character:" (or "char:").
The copyright type indicates the tag represents a game, a novel, an anime, or some sort of copyrighted setting. Otherwise they work identically to character and artist tags, only you prefix with "copyright:" instead (or "copy:").
Species tags identify the tag as a species. Examples are fox, wolf, canine, thylacine, and lagomorph.
General tags are tags that have no other type. This includes common tags like nude, male, and monochrome. Set a tag as a general tag by prefixing it with general: