Hi...
A Mask layout and Drawnt Layout mainly have similar kind of design methods, however the difference lies in the stage of fabrication where these are used.
A mask layout is used at various stages in fabrication say from deposition to well formation,diffusion ,gate formation etc.
Whereas a drawn layout is used as a starting document in fabrication. After the design is complete in frontend, the layout is what is required to be made to start the fabrication process.
Please refer to Mordern VLSI design by Wolfe for more details...thanks