Sonic boom is the auditory phenomenon associated with shock waves created by an object which is travelling faster than the speed of sound.
An object, when flying through air, generates pressure waves in front and behind itself which propagate at the speed of sound (approx. 1225 km/h at sea level and 20°C). If the object is moving fast enough new pressure waves during the movement are generated, at a faster rate than the propagation speed of the previous waves, leading to a merging of the wave-fronts; eventually if the flying object moves faster than the speed of sound it will lead the advancing wave-front and the various pressure waves will merge in a single shockwave, creating a cone-like figure known as Mach Cone.
Someone who is observing an object flying faster than the speed of sound will therefore see first the object, and then will be “hit” by the shockwave - which he will experience as the sonic boom.
The most common - and awesome - examples of this phenomenon are supersonic jets, which generates Mach Cones when reach (and surpass) the speed of sound: the power (ie. the volume) of the shockwave depends on the quantity of air that is accelerated and compressed by the pressure waves - thus depending on the size and shape of the jet - while the grey cone you are going to see in the photos is actually formed by condensed water droplets, resulting from the shockwave shedding from the jet.
Air Force F-22 Raptor executes a supersonic flyby over the flight deck of the aircraft carrier USS John C. Stennis. Gulf of Alaska, 22/06/2009. U.S. Navy photo by Sonar Technician (Surface) 1st Class Ronald Dejarnett;
An F/A-18C Hornet assigned to the Wildcats of Strike Fighter Squadron (VFA) 131 breaks the sound barrier during an air power demonstration above the aircraft carrier USS Dwight D. Eisenhower. Mediterranean Sea, 9/07/2010. U.S. Navy photo by Mass Communication Specialist 3rd Class Bradley Evans;
An F/A-18C Hornet, from the “Argonauts” of Strike Fighter Squadron (VFA) 147, breaks the sound barrier while making a high-speed pass close to the Nimitz-class aircraft carrier USS John C. Stennis. Pacific Ocean, 24/08/2007. U.S. Navy photo by Mass Communication Specialist 2nd Class Ron Reeves;
An F/A-18C Hornet assigned to Strike Fighter Squadron (VFA) 113 breaks the sound barrier during an air power demonstration over the Nimitz-class aircraft carrier USS Carl Vinson. Pacific Ocean, 6/06/2011. U.S. Navy photo by Mass Communication Specialist 3rd Class Travis K. Mendoza.