Also just to keep it accurate as far as the books went. Warlock's are normally distinguished by a strange characteristic not really a symbol. [@dragonmancer] Something like scaly skin, cat eyes, horns, these are the marks of a warlock in this world. They are generally the spawn of a demon and human and can be quite old. One of my all time Fav Char's from the first series was Magnus Bane a really awesome, bisexual warlock. Ill go find a Wiki, im sure it will help more, ill update this post so you can see it Mancer. 

[hr]

Here is a link to [url=http://shadowhunters.wikia.com/wiki/Warlocks] Warlocks [/url] and one to [url=http://shadowhunters.wikia.com/wiki/Magnus_Bane] Magnus Bane [/url] cause he's really awesome and should give you an idea of what they normally do.