The jet stream is a column of extremely fast-moving air in the middle layer of the Earth's atmosphere (specifically the tropopause); it is about 6 to 9 miles above the surface, and blows at a speed typically ranging between 120 and 250 miles per hour. Actually there are a few different jet streams, most importantly one in the north near the poles and two more at the tropics. Jet streams are driven by the difference in temperature between major air masses like the cold polar air mass and warm tropical air mass.
Most clouds are below this altitude, but the huge cumulonimbus clouds that cause storms can reach the altitude of the jet stream. When they do, the jet stream pushes them with an enormous amount of force, making them move much more rapidly across the continent. The jet stream also moves around depending on the season and other factors, and as a result storms are pushed in different directions. The jet stream has been moving north lately, possibly due to global warming; this has caused droughts by moving rainclouds away from places they used to be and floods by moving them where they didn't used to be.
The jet stream also has an indirect effect on weather patterns by influencing prevailing winds at lower altitudes.
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