Shock waves are very thin regions across which the fluid properties change significantly. Shock waves are created when the object travels beyond the speed of sound. Any object traveling in the atmosphere creates pressure disturbances. When the object travels beyond the speed of sound it creates a stronger pressure disturbance. These pressure disturbances pile up and merge to form a shock wave.
Shock waves are oblique shock waves, normal shock waves, bow shock waves, and expansion waves. Oblique shock waves are oblique to the flow direction. Oblique shock waves are created by a sharp nose body, like a wedge. Normal shock is normal or perpendicular to the flow direction. A normal shock wave occurs in front of a detached shock wave.

Oblique shock wave

A shock wave which is detached from the object is a bow shock wave. A bow shock wave is created by a blunt body. Bow shock waves are curved.

Bow shock wave around a blunt body

Expansion waves are formed when supersonic flow turns over a convex corner. Expansion wave consists of an infinite number of Mach waves.

Expansion wave at a convex corner

Across a normal and oblique shock wave pressure, density, temperature increases, and Mach number decreases. Across an expansion wave pressure, temperature, density decreases, and Mach number increases.