Practitioners of the art are called Aikidoka. Equipment consists on a white uniform called Aikidogi very similar to the one used in Judo, only complete with ample black pants known in Japanese culture as hakama.
It was was created not so much as a martial art, but rather as a philosophic movement of universal peace and reconciliation. His creator Ueshiba was trained in judo, Sumo and several styles of jujutsu, but he chose Daito-ryu Aikijujutsu as the core of his new style and gave it the name of aikido in its honor. He stripped it of its most martial traits and added a strong religious background from a neo-Shinto sect named Omoto-kyo, and the result was something that he expected it would attract people in the world to utopia and compassion. It soon became official when the government of Japan started a movement to organize native martial arts and aikido was caught into the set. The art later expanded to United States, Europe and other countries, and although it never gained as much popularity as other, more classic martial arts, its presence can be felt.
Aikido is a controversial one among martial arts, as it could hardly be called a fighting style and absolutely not a combat sport, which is a constant source of confusion and/or discussion. The philosophy of aikido enforces little to no alive sparring or pressure testing, the basis of any effective hand-to-hand training, and instead works on prearranged, compliant drills and katas, often including staged katana attacks and ceremonial elements any Edo period samurai would be familiar with. (Its training does include the word randori, which in judo means alive sparring, but aikido understands it as a sort of semi-improvised exhibition with compliant partners.) Intuitively, this philosphy excludes competitions or championships too, as they are considered to go fundamentally against the art's message of peace and nonviolence.
As for why was aikido designed this way, it is unknown even what were Ueshiba's intentions. Some believe he de-emphasized actual fighting training because he expected all aikidokas to be already proficient in other martial arts, just like himself and most of his immediate apprentices; others believe he did so because he expected his style to be effective without aliveness in training, which brings the additional debate of whether aikido is effective or not (and if not, whether it should be, or how much should it change in order to be); and finally, others don't make such questions because they believe aikido is what it is. In general, aikido is an art with a lot to discuss about, though members within its community are not always open to it.
Although aikido is mostly governed by the Aikikai organization, led by Ueshiba's descendants, it is not a fully unified art, and there are many other small schools and ofshoots. These are the main schools in general.
- Aikikai: the original aikido body and official legacy to Ueshiba's works, currently led by Moriteru Ueshiba, grandson to Morihei. An umbrella organization to many schools and masters, it is the most traditional in terms of style: it follows faithfully the mainstream dogma, composed by what you read above and only sometimes every master's individual exegesis of Ueshiba's teachings. It operates from the Hombu Dojo, aikido's world headquarters, and is represented globally through the International Aikido Federation.
- Yoseikan: it was founded by Minoru Mochizuki, a judoka and karateka who became a direct apprentice to Ueshiba. His eclecticism can be noted in the style's moveset, as it included a lot of elements and techniques from judo, and it was turned at The '70s into a full-fledged martial arts association named Yoseikan Budo that includes judo, karate, kendo and other disciplines. Its style of aikido nowadays tends to be the mainstream one, although its masters sometimes train randori in a judo-esque manner, with full sparring and even some groundwork.
- Yoshinkan: founded by multiple martial artist Gozo Shioda, it could be considered the hardest and most combative of all aikido doctrines, oxymoronic as it might sound. Shioda supplied aikido's lackings with sheer Training from Hell, to the point the Tokyo Metropolitan Police adopted it only because it served well to churn out tough guys. It is famous for its senshusei course, an intensive, 11-month course that everybody can take but which only riot policemen and excellent athletes are expected to finish.
- Shodokan: an oddity between aikido styles in which it has a form of competition. It was created by former judoka Kenji Tomiki (so the art is often called simply Tomiki Aikido) and was naturally controversial among aikidokas from the very start, but it managed to become official and accepted as well. Shodokan matches are structurally one-sided but fully competitive: an aikidoka tries to execute a technique while his opponent tries to resist or counter it with another technique, and they switch places at the next round.
- Iwama-ryu: it was founded by Morihiro Saito after Ueshiba's death, which guaranteed there would be an eternal controversy about this style. Its practitioners claim it is closer to what Ueshiba taught than officially sponsored ones like Aikikai, while detractors call them deserters and reactionaries.
- Ki no Kenkyukai (Ki Society): the last great school, also founded after Ueshiba died. The most mystic of all the aikido schools, devoting actually more time to the control of ki through meditation and yoga than anything else. Unusually, it used to have a form of contest, though not in the vein of Shodokan, but rather as an artistic competition like ballet.
Famous or notable aikido practitioners
- Steven Seagal: despite his personal life and controversies, Seagal is synonymous with aikido in cinema. Not only he popularized it in action films, he remains as the only action/martial arts star who has aikido as his core discipline.
- Sean Connery: an aikido black belt. He famously got his wrist broken before filming Never Say Never Again in a training with Seagal.
- Yayan Ruhian: one of the stars of The Raid Redemption has aikido training aside from pencak silat.
- John Denver: another black belt.
- Jerry Seinfeld: same.
- Joan Baez: same.