Aikido (Ai - Harmony, Ki - Spirit, Do - Way) is a japanese martial art developed by Morihei Ueshiba as a synthesis of his martial studies, philosophy, and religious beliefs. Aikido is often translated as "the Way of unifying (with) life energy" or as "the Way of harmonious spirit." Ueshiba's goal was to create an art that practitioners could use to defend themselves while also protecting their attacker from injury.
Aikido is performed by blending with the motion of the attacker and redirecting the force of the attack rather than opposing it head-on. This requires very little physical strength, as the aikidōka (aikido practitioner) "leads" the attacker's momentum using entering and turning movements. The techniques are completed with various throws or joint locks.
Aikido derives mainly from the martial art of Daitō-ryū Aiki-jūjutsu, but began to diverge from it in the late 1920s, partly due to Ueshiba's involvement with the Ōmoto-kyō religion. Ueshiba's early students' documents bear the term aiki-jūjutsu.
Ueshiba's senior students have different approaches to aikido, depending partly on when they studied with him. Today aikido is found all over the world in a number of styles, with broad ranges of interpretation and emphasis. However, they all share techniques learned from Ueshiba and most have concern for the well-being of the attacker.
Aikido has a history of over 800 years during which the techniques were used and developed on the battlefields and in hostile conditions. Over the years the techniques have been modified and adapted to suit our modern style of living.
Students will not become experts overnight, during the first six months basic exercises are taught as well as breakfalls (ukemi). These are designed to condition ther body whilst practising Aikido. Next students will be shown techniques to control an attack by blending with the attackers strength and redirecting the power against them. By constantly practising these basic techniques, speed and agility will increase allowing students to become more competent in an aggressive situation.
(Some infomation taken from Wikipedia).
Additional Information: Aikido History, Aikido Glossary