The Warlock is a magic-user who is able to deal massive amounts of damage and inflict de-buffs on his enemies. However, the spells he casts requires some time so he is extremely vulnerable during that time.
The Warlock acts as a DPS during dungeon raids. He helps the party through dealing massive amounts of damage from the back of the party.
The Warlock has spells which can make the opponents weakened. This makes him a good class to have on large-scale PvP events. However, the casting time of his skills is his weakness, along with his low Max HP, he is extremely vulnerable to attacks. Which is why he has to remain at the back of the party.