NMOS transistor in layout
The layers that you list look like abbreviations from a CAD system  as such they mean what they are defined to mean within the technology description defined for that process.

However if I had to take a stab at this:-


DIFF is diffusion the whole lightly doped yellow area that forms the substrate.
POLY1 is the Polysilicon area which forms the black bit the gate.
NIMP N+ type implant is the source and drain regions colored purple on your diagram.
PIMP Would be P+ implant which would be used for P channel transistor source and drain this is not shown on your diagram.