An abstract is a general term for a view or description that has only the amount of information required for whatever you are doing. A LEF, or Library Exchange Format, is an example of an abstract. A LEF file contains information about a cell or macrocell for back end design tools to use to do their job. LEF files don't have detailed layer information for every instance. Instead, they contain only the physical properties required to do something like floorplanning and die area estimation. A tech LEF is a LEF file with added information from the technology library. These contain things like design rules and metal blockages. A router can use these tech LEFs to route wires on a chip.
For more information about physical design and file formats, look at chapters 15-17 Application Specific Integrated Circuits by Smith.
Hope this helps.