What is the cause of the jet streams?
The subtropical jetstream
The subtropical jetstream can be explained with angular momentum arguments.  In an idealized circulation model of the atmosphere, there is a hadley cell that circulates upward at the equator, poleward along the tropical tropopause, downward around 30 N/S and equatorward at the surface.  Parcels rising at the equator have a fair bit of angular momentum from the rotation of the Earth.  As these parcels move poleward they tend to conserve this momentum and this manifests as a westerly wind.
The subtropical jet is weak in the summer (ITZC and maxmium solar heating disrupt the hadley cell in that hemisphere), strong in the winter and is generally fixed at around 30 N/S and at a higher altitude than the polar jet.
The polar jetstream
The polar jetstream draws its energy from horizontal temperature gradients in the lower troposphere as a consequence of the thermal wind.  To understand the thermal wind, consider a temperature gradient at the surface, cold toward the pole and warm toward the equator.  Over the cold air, heights of pressure levels will be lower than normal and over the warm air the heights will be higher than normal.  If you look at any pressure level this will result in a height gradient with higher heights toward the equatot.  This gradient will increase with height.  The geostrophic wind is proportional to the height gradient and thus increases with height.  The polar jet exists where these surface temperature gradients are strong but above them around 250 mb.
You will find the polar jet wandering around and associated with the strong surface fronts that come out out of the poles and into the mid-latitudes.  It will be stronger in the winter when the fronts are stronger.  During the summer the jet will remain at higher latitudes and in the winter can dip well into the mid-latitudes.