Jet streams are strong westerly winds generally concentrated in a relatively narrow and shallow above the earth's troposphere. Jet stream's air currents circumnavigate the globe. It has two main jets. The first one is the polar jet which is considered as the strongest jet stream. Another one is the subtropical jet which is a weaker jet compared to the other jet stream.
Jet streams are the aftermath of the combination of a planet's rotation on its orbit and atmospheric heating by solar radiation and internal heat of some planets.