Very quick reply:
If you push the altitude knob you get Climb (CLB) or Descent (DES) modes. These are the managed modes (Open Climb/Descent being the selected modes), and will follow any altitude constraints programmed into the FMC. For this to work, of course, you need to be in 'NAV'.
The RWY mode comes up on takeoff, when you set takeoff power, if there is an associated localiser emitting for that runway. It gives you lateral steering guidance which can be helpful in LVPs.
If you have to fly a heading after departure, you preset this on the FCU on the ground. This gets rid of the 'NAV' in blue on the FMA. Once airborne instead of 'NAV' engaging 'RWY TRK' engages, guiding you along the extended centerline until you decide to pull heading.
Hope that is of help.