What are the key differences between subsonic and supersonic flow?

In subsonic flow (M < 1), disturbances propagate upstream allowing gradual pressure changes, and airfoil shape determines pressure distribution. In supersonic flow (M > 1), disturbances cannot propagate upstream, forming shock waves with sudden pressure and temperature increases. Supersonic flow exhibits wave drag, requires different airfoil shapes (thin, sharp leading edges), and follows different area-velocity relationships (converging ducts accelerate subsonic flow but decelerate supersonic flow).