Jet streams are areas with high wind speeds in the upper troposphere. Intensity, location and shape are highly time dependent. They predominently occur at mid-latitudes, where tropical and polar air masses collide. Jet streams are usually a few thousand kilometres long with a width of some hundret kilometres. By definition you can call high wind fields in the atmosphere a jet stream, if the wind speed exceeds 60 knots (kn), which are 111 km/h.
The figure shows a jet stream on Nov. 8, 2015 at 21 local time at an altitude of 10.5 km above Central Europe. The wind data comes from the numerical weather prediction model, that DreiTageWetter uses for its forecast. The highest wind speeds are above England and the North Sea (yellow sector). There the wind is faster than 140 knots or 260 km/h! The altitude of the highest wind speeds of the jetstream is equivalent to the cruising altitude of airplanes. Therefore the jetstream is important for aviation, especially for lengthy flights such as flights over the Atlantic Ocean. As in mid-latitudes usually westerly winds occur, you can use the tailwind for flights from west to east and you have to avoid the jet stream in the opposite direction.