If you have the autothrust engaged, then there are 3 detents - Climb, Max Continuous/Flex and TOGA. If you take the Autothrust out, which you can do at any time in the flight, then the thrust levers work just like the T/Ls on any other aircraft.
To explain the 3 detents a little more, since this is what you're asking about - at take off, the T/Ls are placed either in TOGA if you're doing a max power T/O, or in Flex/Max Continuous detent if you're doing a Flex power T/O. When the levers are in either of these detents the FADEC will give you the max power available for that selection (It depends on the ambient conditions). When you reach Thrust Reduction altitude you move the levers backwards to the Climb detent, and you will then get Climb power. The levers remain in this detent until you reduce thrust to idle in the flare during the landing - this means that the FADEC will give you whatever power is necessary to achieve what you are trying to do - climb power for the climb, or whatever thrust is necessary to achieve your cruising Mach number, or desired rate of descent or whatever.
If an engine fails, then the levers on the remaining engines are moved back into the Flex/Max Continuous detent, and you will get Max Continuous power. If you have to do a go-around at the destination, you put the levers into the TOGA detent and you get all the power available.
This is all pretty standard stuff for an Autothrust system, the main difference between Airbus's system and Boeing's is that on a Boing the levers move constantly as the FADEC adjusts the power, whereas on an Airbus the levers remain static unless the pilot moves them as described above - to see what the FADEC is doing in an Airbus you have to monitor the N1 gauges.
As I said in the beginning, the pilot can deselect the Autothrust and move the levers manually at any time if he feels like it.
Hope that all makes sense - have just returned from a night spent flogging across the pond so my brain is not too active at the moment.
Ah, have just read your question about what you do if you're too low on approach and need more power. If you leave the Autothrust engaged it will (eventually!!) add the required power - that's what Autothrust systems are for. If you've disconnected it for the approach (some of us do sometimes, just ot prove to ourselves that we still can
) then you add the needed power yourself - just like any other aeroplane.
(Edited to add the last paragraph).