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.