Aikido is a modern Japanese martial art (Budo) that was developed by Morihei Ueshiba (O-Sensei).
Aikido is the way of harmony. In Aikido we seek to reach a peaceful resolution from conflict; in day-to-day practise we do this by blending (or harmonising) with the energy of an attack and pinning or throwing our partner(s) in a safe and controlled manner. Aikido practise involves coordination / relaxation techniques, breathing exercises and weapons exercises to enhance 'Ki'; a Japanese word meaning energy or spirit.
As the techniques involve using your partner's energy you do not need to be physically powerful to effectively practise Aikido.
Some time later Ueshiba encountered a religious leader named Onisaburo Deguchi of the Omoto Kyo sect, it was he who encouraged Ueshiba to begin teaching the martial arts he had learned, and to teach them as a spiritual pursuit rather than a way to be only physically stronger. After a time Ueshiba became a well known martial arts teacher naming his art Aikido, meaning 'way of harmony', in 1942. He eventually retired to live and teach in the small country town of Iwama leaving his only son Kisshomaru in charge of running the organisation he had set up to spread the teaching of Aikido throughout the world, this organisation was called the Aikikai and its chief instructor was a man named Koichi Tohei (Tohei Sensei was awarded the highest possible rank in Aikido, 10th Dan, shortly before O Sensei's death).
O Sensei passed away on the 26th of April 1969 leaving his son in charge of the Aikikai, however, in the years directly after World War II there had been chaos in the then occupied Japan and teaching Aikido was difficult at best (especially seeing as the occupying Americans had banned all martial arts), during this time a few of O Sensei's more senior students began teaching independently of the Aikikai, this would eventually lead to them developing into new styles of Aikido, namely Yoshinkan Aikido under Gozo Shioda Sensei and Shodokan (sometimes called Tomiki) under Kenji Tomiki Sensei. In 1974 the chief instructor of the Aikikai, Koichi Tohei resigned his position in order to begin his own organisation Shin Shin Toitsu Aikido, which focused on teaching Aikido with mind and body coordination principles, often referred to as Ki-Aikido in the west.