What is the difference between ideal stages and transfer units in a packed bed gas absorber?
A transfer unit gives the change of composition of one of the phases equal to the average driving force producing the change
Source: http://www.separationprocesses.com/Absorption/GA_Chp04c.htm

So to phrase it differently, for some ideal zone in the absorber that has a particular value to the average mass transfer driving force, there will be a fixed change in the phases present at that ideal zone.
NTU is somewhat analogous to the total number of theoretical stages if the operating and equilibrium line are straight and parallel.
There is no direct physical interpretation of what a transfer unit. If you design any appreciable size packed bed tower, you will find that the vendors of the packing will use a lot of empirical correlations for their own products to determine separation efficiencies and other related parameters.