Droop is the characteristic curve of a diesel generator.
Droop is the curve plotted of the KVARat the x-axis and the
KW in the y-axis
Reverse power in an alternator happens when the unit is not
properly synchronized with the bigger power system that your
unit is interconnected. Reverse power may result in tripping
your Air circuit breaker protection or in worse case cause
the alternator to burn or the engine drive to rotate in
reverse and result to damages

droop is a characteristic of  a generator. It  indicates 
load sharing pattern of that particular machine. base load 
station will have  less droop,i.e. its generation will not 
change  much to changes in grid frequency. but peak load 
station wil have higher droop, as it requires to absorb 
more load  when demand increases.( frequency drops)

droop characteristics in a diesel generator refers to a
particular type & make of an engine which is unique for that
make and type.if we have to synchrnise two dg sets, they
should have same or similar droop characteristics so that
they respond to the load in a similar and proportinate
manner according to their capacity and uniform time.droop
characteristics depends on type of engine,governor,etc

Droop characterstics of an alternator are
(a) load vs frequency/speed ie effect of frequency change on
the load sharing of an alternator
(b) effect of changing ecitation of armature on the terminal
voltage of the alternator