NBS generates bromine radicals that will potentialy substitute any easily abstractable hydrogen. Therefore it is most usefull for the allylic position (alpha to the double bond) and benzylic but sometimes also other positions as well if there is nothing better.
Bromine behaves like an electrophyle (except in special conditions: UV or peroxide) and is therefore useful for elctrophilic substitutions and additions: substitutions of aromatic hydrogens, addition on the double/triple bond...