LEF - Library Extraction format....
These are the file which contains physical information abt libraries. These are the inputs to encounter tool to know abt libraries.
DEF - After routed netlist, we can also save the option, File - Save - DEF.....
This DEF is going to input for generation of SPEF in Celtic tool.
May be some other reason also ther for DEF.
Def is nothing but it contains place and route information.When we do floorplan , placement and routing the file wil be saved in DEF format
Hope it is clear for u
And LEF contains design rules which is given by the foundry and that is nothing but our library...
Like it contains rules for the metal layers and so on
If you are a front end person, this is the analogy -
LEF is the physical equivalent of library db (or adb if you use buildgates). DEF is the physical equivalent of design db (or adb if you use buildgates).