Field:

A field in an AC generator consists of coils of conductors inside the generator which receive a voltage from a source (known as excitation) and generates a magnetic flux.  In the field cuts The magnetic flux the armature to produce a voltage. This voltage is ultimately the outcome voltage of the AC generator.

Armature

The armature is the elements of an AC generator in that voltage is generate.   This component consists  of  several  coils  of  wire  which  are  huge  enough  to  carry  the  full-load  current  of  the generator.