EXE is the common filename extension denoting an executable file (a program) in the DOS, OpenVMS, Microsoft Windows, Symbian, and OS/2 operating systems. Besides the executable program, many EXE files contain other components called resources, such as bitmaps and icons which the executable program may use for its graphical user interface.
16-bit DOS MZ executable: The original DOS executable file format, these can be identified by the letters "MZ" at the beginning of the file in ASCII.
16-bit New Executable: Introduced with Multitasking MS-DOS 4.0, these can be identified by the "NE" in ASCII. These never became popular or useful for DOS and cannot be run by any other version of DOS, but can usually be run by 16/32-bit Windows and OS/2 versions
32-bit Linear Executable: Introduced with OS/2 2.0, these can be identified by the "LX" in ASCII. These can only be run by OS/2 2.0 and higher. They are also used by some DOS extenders.
Mixed 16/32-bit Linear Executable: Introduced with OS/2 2.0, these can be identified by the "LE" in ASCII. This format is not used for OS/2 applications anymore, but instead for VxD drivers under Windows 3.x and Windows 9x, and by some DOS extenders.
32-bit Portable Executable: Introduced with Windows NT, these are the most complex and can be identified by the "PE" in ASCII (although not at the beginning; these files also begin with "MZ"). These can be run by all versions of Windows and DOS (DOS runs the MZ section, Windows runs the NE or PE section). Using HX DOS Extender DOS can load the NE and PE sections. They are also used in BeOS R3, although the format used by BeOS somewhat violates the PE specification as it doesn't specify a correct subsystem. These can also be used on ReactOS
64-bit Portable Executable (PE32+): Introduced by 64-bit versions of Windows, this is a PE file with wider fields. In most cases, you can write code that simply works as either a 32 and 64-bit PE file .
Besides these, there are also many custom EXE formats, such as W3 (a collection of LE files, only used in WIN386.EXE), W4 (a compressed collection of LE files, only used in VMM32.VXD), DL, MP, P2, P3 (last three used by Phar Lap extenders), and probably more.
When a 16-bit or 32-bit Windows executable is run by Windows, execution starts at either the NE or the PE, and ignores the MZ code. On the other hand, DOS cannot execute these files (except using HX DOS Extender, which supports PE files only). To prevent DOS from crashing, all Windows executable files should and usually do start with a "working" DOS program called a stub.,simply displaying the message "This program cannot be run in DOS mode" (or similar) before exiting cleanly. A few dual-mode programs (MZ-NE or MZ-PE) (such as regedit and some older WinZIP self extractors include a more functional DOS section).