http://www.cnblogs.com/laynelin/archive/2009/12/31/1636767.html
The distinction between EXE and DLL files is entirely one of semantics. They both use the exact same PE format. The only difference is a single bit that indicates if the file should be treated as an EXE or as a DLL. Even the DLL file extension is artificial. You can have DLLs with entirely different extensions-for instance .OCX controls and Control Panel applets (.CPL files) are DLLs
--摘自An In-Depth Look into the Win32 Portable Executable File Format
就是一個比特位的不同，告訴OS究竟是exe還是dll
