its design exchange format. it conveys the physical design information. like i said, if u r using a p&r tool, if u floorplan and do placement, u can save it in def format. this can be imported again to complete the rest of the steps.
.lib: cell timing, area, power info
.lef: technology info + cell geometry info
.def: design netlist + geometry info
.tf: technology info, which is only for synopsys usage.
cadence uses: .lib, .lef, .def for library and design
synopsys uses: .lib, .tf, .pdef, Milkyway for library and design.