DLL files can also be used by more than one program. In fact, they can even be used by multiple programs at the same time. Some DLLs come with the Windows operating system while others are added when new programs are installed. You typically don't want to open a .dll file directly, since the program that uses it will automatically load it if needed. Though DLL filenames usally end in ".dll," they can also end in .exe, .drv, and .fon, just to make things more confusing.
File extension: .DLL