Aikido

Aikido is the thinking man's martial art.  The concept of aiki involves blending with an attack to control and neutralize it, rather than opposing it directly.  In this way Aikido's goal is to gain control of a situation, taking minimal damage, and using minimal force to do it.  This has made Aikido a popular form of self defense with people of all sizes.  

Aikido emphasize leverage, soft circular motions and, when performed correctly, allows for the restraint or incapacitation of an assailant (or multiple) with complete control (and, thus, without hurting the attacker--you merely suppress their ability to cause harm).  

History

In the grand scheme of things, Aikido is a baby brother to many traditional martial arts, but demonstrations can illustrate how dynamic it can be.  Created in the early 1900's by Morihei Ueshiba (who is referred to by aikidoka (aikido practicioners) as O'Sensei), Aikido is based largely on several older styles, primarily aiki-jujitsu and even including judo and spear techniques.  

Circles

Many techniques in aikido follow very fluid, often circular curves.  There are a number of 'Circuits of Neutralization' which generally lead an attacker down into a joint-locked position (controlled by leverage applied by the aikidoka), preventing further movement.  

Thus, an attacker's energy is met (blended with), and then directed either away or down (into a Circuit of Neutralization).  

Ki

Very prevalent in Aikido is the concept of Ki.  There is much disagreement about whether ki is a supernatural force (energy) or a harmony of perfect technique (or something in between) but Aikido can yield dramatic results.  It is said that Morihei Ueshiba could cause a person to fall without touching them (simply by holding his hand near their head and moving to 'change the direction of their ki').  

Cautions and Criticisms

Aikido, when performed properly, does not hurt the attacker, but merely controls him (or her).  However, critics point out that because of aikido's training philosophy (traditionally, the partner in practice does not resist the moves), while real-world attackers will resist, and thus aikido training may not prepare one adequately to a real fight.  This is often dealt with by decreasing cooperation and increasing resistance (by more skilled partners, after a sufficient level of control has been reached), and it has also been pointed out that while most martial arts techniques cannot be practiced fully (and must be cut short or simulated to avoid injury to a partner), Aikido techniques can be practiced fully without injury.