Madougu are special weapons used by the Hokage ninjas more than 400 years ago. There are mainly two people who created them, Kokuu and Kaima. Kokuu created the good madogou while Kaima created the evil counterparts.
Joker's madougu is the Taishaku Kaiten (帝釈廻天). The Taishaku Kaiten is a trident looking madougu with a huge eye on the trident end and a sharp axe-like weapon on the other end. One of the most difficult madougu to master, Joker could manipulate gravity over a specific area with it.
By reducing gravity, it's possible that Joker could reduce the opponent's attack damage to zero. This made him look like someone with superhuman strength since he could stop an attack with just one arm like what he did with J-Keeper's attack. Also, reducing gravity allows him to make heavy things lighter. That's the reason why he was able to lift and throw Mikoto with his trident with only the use of one arm. This also allows him faster movement. On, the other hand, aside from reducing gravity, he could also increase it, thus make things heavier. If he invokes the hyper mode of this, meaning he increases the area of the gravity field, it would render projectile attacks useless since projectile weapons would fall to the ground even before reaching him. On the other hand, instead of increasing the area, if he keeps on increasing gravity within a small area, it's possible that he could open a blackhole. This is how the fight between him and Shishiten's Kadotsu ended. He opened a blackhole that sucked both Kadotsu and him in.
Aside from controlling gravity, the Taishaku Kaiten will also act like a normal lance and ax weapon that could stab, slice and dice Joker's enemies. There are time that he also uses it as a projectile weapon, throwing it towards his opponent.