Why is the Polar Front Jet so called, and why is it important to us in NW Europe?
In NW Europe, when meteorologists refer to a jetstream, it's the Polar Front Jet (PFJ), that is usually meant. As its name implies, it is associated with the classical 'Norwegian model' polar front - the surface discontinuity between cold/ex-polar latitude air, and the warm, relatively moist air originating in the sub tropical anticyclone belt.
When air masses (see "When was the concept of an air mass proposed?" and "So, how is an air mass defined?") lie adjacent to one another, the temperature difference isn't just found at mean sea level, but throughout the troposphere. Because atmospheric pressure decreases more quickly with height in cold/polar air than warm/sub-tropical air, there arises a pressure differential, which gives rise to intense pressure gradients at altitude, and hence the very strong winds observed. Because of the high wind speeds involved with jetstreams, any slight changes, in either velocity or direction, or both, leads to vertical motion in the air below the jet, and is a major player in the processes of atmospheric development.
(For more on upper air meteorology, jetstreams etc., see "Upper Air Meteorology").