Elevated ways are built over other ways, not too high buildings, or just on the ground by the same way as usual ways.The cursor can be placed on an elevated way so it's possible to build stations, depots and signals on these ways.
You can link elevated ways with ground ways with bridges, some elevated ways have an access bridge just for this use, otherwise you have to use a normal bridge, select the bridge tool and click on the ground way's extremity :
It's possible to stack elevated ways even if it's graphically not good, however manage the link with the ground is a bit difficult …
Build a normal bridge or a tunnel on an elevated way is impossible, you have to use normal ground ways in some cases as this bridge :
Next tutorial : Stations