A title is decorative text displayed above your character's name and below your guild's name. To change your title and see which are available to you, open your Log window (Press L), click on the Fame tab at the top of the window, click on a title in the bottom-right window section, then click on the Set button. You can also turn off seeing everyone's title (including yours) by going into the Options window, click on the Other tab, un-checking the Title box, and clicking OK.
Since the new patch, different titles come with different attributes and come in different colours. Just like equipment green titles are lower and yellow titles are higher.