A DLL is a file of program code stored on disk that an application loads as and when it needs it. It means that it's not stored in memory unused; rather it is kept on disk until it's needed. Large applications, such as a complex word processor, might use several DLLs: one to carry out the spell-check; one to manage printing a letter; and a third for formatting the text. DLL files have a .DLL three-letter extension and so are easy to spot using the Windows Explorer. Windows itself uses DLLs to carry out tasks that are only occasionally required. Windows stores its DLL files in the System sub-directory.
