Referred to as the hard style, Yoshinkan Aikido style is a very direct form of Aikido with the emphasis on basic movements and practical self defense. Yoshinkan Aikido is the only officially recognized form of Aikido by the Japanese government and is taught to the Tokyo Police Force.
What is Aikido?
Aikido was created by the founder Ueshiba Morhei in the 20's in Japan. Aikido was the founders' idea of taking ancient Samurai techniques and making them more effective by using the attacker's energy against them. Combining the hand to hand techniques of jujitsu and the body movement of Samurai sword fighting, the founder created a martial art today that is incredibly practical and a favorite of martial artists all over the world.
More info: Aiki Wiki
Gozo Shioda was a student of O'Sensei pre WWII and took his mastery of preWar Aikido and started his own style "Yoshinkan Aikido"
More info: Yoshinkan Wiki