Moloch was a crime boss in the 1940s through 1960s. He battles both Nite Owls ( Hollis Mason and Daniel Dreiberg), Dr Manhattan, and Ozymandias. He then comitted crimes in less conspicous ways but still met defeat. He eventually gives up on crime. Moloch worked at Dimensional Developments (a company owned Veidt aka Ozymandias) where he was secretly bombarded with radiation to induce cancer as a part of a plan to exile Dr. Manhattan.
As explained in Before Watchmen, Moloch was born deformed and unloved by his parents or peers. He became obsessed with magic which allowed others to be entertained by him and ignore his deformity. Eventually he fell in love with a local girl, and after being tricked by her he killed her boyfriend, dumping the body on her bed and then he ran away from home. He became a stage magician but after a lack of success he turned to crime using his magic tricks to confuse police. He soon gained power and started his own criminal empire. When the MinuteMen were formed he ended up clashing with them many times. His plans fell apart upon the appearance of Dr.Manhattan who's powers allowed him to easily defeat Moloch. Moloch repented after facing Dr.Manhattan's godly powers and eventually got a job working Ozymandias on a fake project.
The Comedian broke down in front of Moloch one night (while Moloch was in bed). He explained to Moloch what he knew about the conspiracy. However, due to his drunkeness, Moloch did not understand Blake's intention. This led to Blake's death as Vedit had bugged Moloch's appartment. Rorschach ruthlessly interogated Moloch and found out about their meeting before Blake's death. Rorschach instructed him to leave a note at a certain location if he found out anything of importantance. Veidt ended up killing Moloch with a bullet to the head and framed Rorschach for the crime and was apprehended by police after briefly fighting the police force sent to arrest him.
While he had no powers he was a ruthless criminal mastermind, effective planner and organizer. He also had a "solar mirror weapon" which the Minutemen had in their possession though how powerful the weapon was or if it even worked is unkown. He also carries a firearm hinting that he could be a good marksman however this can't be proven.
The name Moloch comes from ancient Hebrew or Arabic and means either a god or a certain type of child sacrifice using fire. Moloch also appears in the Bible as a Prince of Hell, known for kidnapping children. It is possible, given the history of the name and Moloch's career as an owner of a vice den that Moloch was a sex offender or was involved in some way with the exploitation of children. Given Rorschach's history with child abusers, this would make sense in the context of the Watchmen graphic novel. Though Moore and Gibbons never said who Moloch was based on there are some characters believed to his inspiration. They are Joker (prince of crime title), Monocle (similar names), Sinestro (both have a satanic appearence), Mysterio (trickery) and Abra Kadabra (magic).It is also possible all of them was the inspiration for him.
Matt Frewer played Moloch in the movie.