DXF stands for Data eXchange Format and belongs to AutoCAD. The file should be output in ASCII format (a binary version exists but is rarely used....)
Each version of AutoCAD outputs a slightly different flavor of DXF but the latest version of AutoCAD, release 13, resulted in a greatly changed and much more complex DXF file. Many applications that previously imported DXF fail on the DXF from version 13.
Many applications output DXF also including virtually every CAD program. However they don't necessarily export all entities. Most do not export AutoCAD's polyline entity and many lose linewidth information.