Aikido was developed by Morihei Ueshiba (14 Dec 1883 - 26 Apr 1969), referred to by some aikido practitioners as O Sensei ("Great Teacher"). Ueshiba envisioned aikido not only as the synthesis of his martial training, but also an expression of his personal philosophy of resolving conflict peacefully.
Aikido finds its roots in the koryu (old-style martial arts) of feudal Japan. Ueshiba dedicated his life to discovering the nature of true "budo" or "warrior's way." By his 40's, Ueshiba was already a renowned master of kenjutsu (Japanese swordsmanship), spear and staff arts, and Daito-ryu aiki-jujitsu. However, mastery of these arts left him unsatisfied, largely because victory always meant death or serious injury to one's opponent. Ueshiba eventually concluded that the "true warrior's way" meant that one should strive to resolve martial conflict peacefully, by blending with an attacker's energy and redirecting it in a non-destructive way.
Aikido techniques include infinite variations, but all share common principles. An opponent (uke) attacks with energy intended to cause harm. Nage (person being attacked) redirects the energy in such a way as to unbalance the attacker, and then applies a throw, pin, or joint lock to neutralize the aggression and gain control. Aikido practitioners spend equal time training to both perform and receive techniques. The graceful falls seen in an aikido demonstration are not the result of uke "cooperating," but rather of uke's ability to receive the full-force technique safely while maintaining martial readiness and posture.
Aikido is currently practiced in thousands of dojos world-wide. Various organizations differ in some respects, but all share the same core principles. Aikido's emphasis on gaining control of an attacker by causing minimum harm makes it a popular martial art for law enforcement and military professionals.
"Even the most powerful human being has a limited sphere of strength. Draw him outside of that sphere and into your own, and his strength will dissipate." - M. Ueshiba