Think of a boat -- or even a duck or goose -- that moves across a lake's surface faster that the wave speed. A speeding boat creates a "bow wave" that goes out behind it in a distinctive "V".
This "bow wave" is because waves bunch up or pile up on each other before they can move away. For a boat on water, this "bow wave" is just an increase in the height of the water.
For an airplane going through the air, such a "bow wave" is an increase in the pressure of the air. This "bow wave" is called a "shock wave". When it encounters something like people on the ground it is heard something like a thunder clap. When it encounters something like a building on the ground, it may rattle -- or break -- windows. For this reason, supersonic flights are usually restricted to uninhabited areas -- such as the oceans.
Both Air France and British Airways fly the Concorde on commercial flights. The Concorde flies above Mach 2 -- faster than twice the speed of sound. Air France and British Airways use the Concorde on flights that are primarily over the oceans so that the resulting sonic boom does not harm people.
(C) 2003, Doug Davis; all rights reserved