The Joker was a supervillain who considered himself the archenemy of Batman. He was described as a homicidal maniac by both Batman and Batgirl, and was admitted into Arkham Asylum many times. He was one of Batman's most dangerous enemies and a dangerous one of the Justice League.
The man who would become the joker was a crook that fell into a drainage vat of chemical waste that washed him out into the river. The chemicals had an adverse effect on him: they dyed his hair green, his skin was bleached white, and his lips were permanently stained red, giving him the appearance of a clown. All this snapped the crook's already twisted mind and sadistic nature, giving birth to the Joker.
A frequent inmate of Arkham Asylum, the Joker was often psycho-analyzed at one point by Dr. Harleen Quinzel. After one session, the Joker fooled Dr. Quinzel as he painted himself as a misunderstood soul crying out for the world to accept him. Quinzel soon found herself in love with the Joker, and broke him out of Arkham.
However, a combination of the Joker's ego and Harley Quinn's lack of humor tends to push the Joker's buttons. More often than not, the Joker abuses and throws Quinn onto the streets until he decides he wants her again.
The Joker possessed little more than average physical strength, albeit enough to regularly hold his own against Batman.
This is the "Batman: the Animated Series" (BTAS) The Joker figure which came with "Laughing Gas" spray gun