The inlet functions to capture and decelerate air prior to entry into the compressor. While the inlet is often optimized for cruising conditions, it must provide adequate massflow during all other engine operating conditions including takeoff, landing, and maneuvering. Some inlets incorporate blow-in doors to provide additional airflow during high thrust conditions at takeoff. Inlet efficiency is generally characterized by stagnation pressure recovery - a measure of the available energy in the air that actually makes it into the compressor.


At subsonic speeds, the ideal inlet is a "pod" or "pitot" installation, as seen on most modern jetliners. This makes full use of the "ram effect" and has minimum size, weight, and effect on the aircraft's aerodynamics.
For smaller single-engine aircraft, a pod generally isn't feasible. Instead, the usual practice is to bury the engine in the fuselage and have one or more intakes on the top, bottom, or sides. Most military jet trainers and many front-line fighters like the F-16 use such an installation. The problem is that if the aircraft maneuevers "away from the duct" (due to yaw or angle of attack) the airflow in the duct is much decreased due to separation.
Neither of these types of inlets do well at higher supersonic speeds, due to shockwaves that form on the lip of the inlet, reducing the efficiency of the inlet.
At lower supersonic speeds, one type of inlet to use is an internal/external compression intake, which produces a series of mild shock waves to reduce flow speed to subsonic without much energy loss.
At higher supersonic speeds, due to the nature of shock waves, the best inlet must have a variable geometry. These often produce one or several oblique shocks, as well as a normal shock, to reduce the flow to subsonic speed. They also have the ability to aid in compresion, and in the case of a ramjet do ALL the compression. Examples of this kind of inlet can be found on the F-15, the Concorde, and many other aircraft.
One other type of variable-geometry supersonic nozzle combines the internal/external compression nozzle with a mobile cone to achieve a similar effect. Nozzles of this type can be found on several older Soviet fighters like the MIG-21, as well as on the SR-71 Blackbird.

Turbine Engine Basics : Compressor