A Voith Schneider unit is like vertical blades that spin around on a disc in the bottom of the hull. The blades are all adjustable so can be either in a zero pitch position where they all rotate on a circle giving no thrust or they can be adjusted so that as they go past a certain point on the rotation that change pitch and give a thrust.
This unit is very quick to respond and can give equal thrust throughout 360 deg. It is not very efficient for normal propulsion though.
An azipod unit is a pod that azumuths!! Seriously it is a motor and prop unit mounted outside the hull which can rotate through 360 deg. The QM2 has two fixed pods and two azimuthing pods. Very efficient and very manouvreable but are still prone to leakage issues. Also the QM2 has damaged one which on a normal ship would require a prop change but on her it necessitates the complete pod change which will take two dry docks and a few months of reduced speeds.
Triple expansion is a steam engine that uses exhaust steam from the first cylinder to power the second cylinder and the exhaust from the second cylinder to power the third one. This is designed to extract the maximum possible energy from the steam. The givaway when you look at one is that the cylinders get larger each time as the steam has expanded that bit more during each working stroke.
Nigel,
Have a look at this link for a more comprehensive explanation:
http://www.voithturbo.de/vt_en_pua_m...spropeller.htm