Guild tags are used in the game to identify which guild (if any) a specific character is in. Guild tags are a combination of characters, up to four, that will show up after a character's name in the viewable area of the game. They also have a color-code available on them. For example, guild members from the same guild will have their tag in a different color (gold) than the other players in the area. Players that are in another guild that is in the same alliance will have their guild tag showing in orange (the same color as the alliance chat text). This effect seems to only apply when in towns and outposts. Guild tags must be between 2 and 4 characters.
The same apparent guild tag can and often does refer to more than one guild.