LEF & DEF & GDSII

LEF:

Definition:
A Library Exchange Format (LEF) file contains library information for a class of designs.
Library data includes layer, via, placement site type, and macro cell definitions.

Place and route tools do not require the full cell layout.
Another view of the standard cells called the abstract view needs to generated
The abstract view provides information like:
1. Cell name, site name, cell orientation„
2. Cell PR boundary
3. Pin names, locations, pin metal layer, type and direction (input/output/input-output)
4. Also provides location of all metal track and vias in the layout (obstructions)
This information is passed to the P & R tool in the LEF format

LEF & Milkyway 
When an outside source provides library cell information in LEF format, you need to read the data file into the Milkyway Environment and convert the data into a cell library in the Milkyway database.

DEF:
A Design Exchange Format (DEF) file contains the design-specific information of a circuit and
is a representation of the design at any point during the layout process.
The DEF format defines the elements of a specific design that are related to physical layout, including the placement and routing information, design netlist, and design constraints.

DEF conveys logical design data to place-and-route tools, and get physical design data from place-and-route tools.
Logical design data can include internal connectivity (represented by a netlist), grouping
information, and physical constraints. Physical data includes placement locations and
orientations, routing geometry data, and logical design changes for backannotation. Placeand-
route tools also can read physical design data, for example, to perform ECO changes.

GDSII:

The GDSII stream format is an industry-standard data exchange format for integrated circuit layout information. A GDSII file contains information about layers, wire paths, boundaries, structures, arrays, and text labels in the cell or chip layout.
A GDSII or OASIS stream file contains physical information about library cells, but it does not specify the cell types.
Each geometric object in a layout database has an associated layer number and data type number. In the GDSII and OASIS formats, the layer number and data type are integers ranging from 0 to 32767.

GDSII & Milkyway library

read_gds:
When an outside source provides library cell information in GDSII format, you need to read the data file into the Milkyway Environment and convert the data stream into a cell library in the Milkyway database.
In addition to the GDSII file, you typically provide a cell-type definition file and a layer mapping file to specify how to translate the data.