You can gain a level in this skill through maxing out the Baby Farmer job. It can be ground through burning people with cigarettes and doing other such sadistic things.
As of very recently, the torture skill gives you a bonus to critical hit chances for all melee weapons that deal:
It also buffs the following damage types that are currently not available for player melee weapons:
[zotnet] Necanthrope says, "energy damage (excluding radiation types), acid, slashing, stabbing all get buffs from torture" [zotnet] Osiris asks, "Curiously, does cold count as energy damage?" [zotnet] Necanthrope says, "yes. energy is: burning, electric, radiation, irradiation, cold, laser, scalding"