Rokushiki is a special, superhuman martial arts style. In the Viz Manga and FUNimation dub, they are called the Six Powers. They each made their appearances in different chapters and episodes, but the technique made its debut when one of the moves appeared in Chapter 342 and Episode 242. It was first introduced being used by the CP9 agents.
The strength of a Rokushiki user is measured by Doriki, which Fukuro can use his "Te-awase" to calculate with. The higher the level, the stronger each technique's potency becomes. Most of the users have their own variation of each technique, to increase their versatility and originality. Also, certain users tend to be more specialized in one technique, such as Fukuro with Soru, Jabra with Tekkai, Kaku with Rankyaku, and Rob Lucci with Shigan.
Rokushiki is the main style of combat for the World Government's secret CP9 agents, but some Marines have also demonstrated its usage. Coby used Soru against Monkey D. Luffy,Vice AdmiralsMomonga and Dalmatian used it to confront Luffy at Marineford, Momonga also used Geppo against Luffy, and Dalmatian also used Shigan against Luffy, Vice Admiral Vergo used it on several G-5 marines on Punk Hazard, and Captain Tashigi used it during her confrontation against Monet. The (non-canon) former Marines Zephyr and Binz are Rokushiki users, with Zephyr being able to master it and Binz seen using at least Soru. It is unknown exactly how much Rokushiki is spread around the Marines and how widely known techniques are, but it seems the most commonly known technique is Soru.
Outside of the Marines, there are also others who learned the Rokushiki techniques, or adapt their principles to create their own, similar techniques. The (non-canon) World Government hating pirate Byrnndi World was somehow able to learn the Soru and Geppo techniques.Monkey D. Luffy utilized his Gear Second mode to grasp the concepts of Soru, gaining an advanced version of it, while Sanji gained an advanced variant of Geppo that he can utilize both in midair and underwater.