A sonicboom is the sound created by an object (generally an aircraft) which breaks the sound barrier, or mach 1, approximately 1,225 km/h (761 mph) at sea level and 20 °C (68 °F). This flight is termed 'supersonic' flight, or speed.
This is caused by the compression of pressure waves (sound waves) behind, and in front of, the object. When the object (in most cases, an aircraft) is travelling at sub-sonic speed, the sound waves are able to propagate ahead of the aircraft, as they are moving faster. But when the aircraft begins to travel at a speed greater than the maximum speed of these sound waves (the speed of sound) the waves are forced to compress together, forming one large pressure wave. The boom is created by the waves that would normally propagate ahead of the aircraft being compressed together while the aircraft flies past them.
Interestingly enough, the crack of a whip is actually a mini-sonic boom, as the tip of the whip moves faster than the speed of sound- no wonder it hurts to get hit with it!