I swiped this excerpt info from a site I knew of :

This Korean Art of Self-Defense is a pure art form and is NOT a sport like many other martial sport types. The fact is, this traditional art was never intended to be a sport, it was utilized as a battlefield art.
The three principles of Hapkido are Yoo (softness, water), Hwa (harmony), and Won (circle theory).
Hapkido (which is a Korean Mu Do) should not be mistakened for Aikido (which is a Japanese Bu Do). Aikido is a passive discipline. Hapkido is a passive (Um) and active (Yang) discipline. There are similarities, but, both are quite different from each other. Also, Hapkido utilizes smaller circles (radii) in executing technique.
The Korean Art of Self-Defense, Hapkido, is considered a "soft" style of Martial Art, in contrast to the "hard" styles that practice the use of brute force-against-force, resulting in size and strength domination. The Hapkidoist redirects, diverts, or suppresses an attackers flow of energy softly and graciously. This diversion or passive act allows the Hapkidoist to use the attackers power against him or herself leading to the attackers defeat or demise. Through the twisting or separation of tendons, ligaments and muscle; application of pressure (cavity press) on specific joints and pressure (meridian) accu-points, very little strength is needed to overcome an attacking opponent.
The Hapkidoist learns to view an attacker as an energy entity rather than a physical entity. In other words, the larger the person that is attacking, the more energy he or she has, the better it is for the Hapkido stylist, and the more devastating it is for the attacker.
Hapkido, defensively, redirects the oncoming attack, and quickly circulates it against the attacker, using small circular, twisting, and angular patterns, that follow through with immediate, joint-immobilizing, joint-locking, accu-point-pressurized, devastation utilizing offensive techniques which are geared to control the attackers violence, rage and render him or her incapable of further destructive action. The Hapkidoist is in total control of the confrontation and thus nullifying the aggressor's attack without the need for uncontrolled damage as seen in many of the "hard" styles.