In the Mortal Instruments a warlock is the offspring of a demon and a human. All warlocks are born sterile meaning they can not reproduce. However in the Mortal Instruments a warlock is immortal and lives forever meaning that the species will not easily die out.
Although the term warlock normally conjures up thoughts of magic, not all warlocks are born with magic. Some warlocks have to go through their life with strange features (explained above) without being able to cast a ward to make themselves look normal.