Dynamic Link Library (DLL) is a file (also called library) which contains executable functions or data that can be used by other applications. A DLL file has the extension "DLL" in its name. A DLL cannot run directly. DLL needs to be called by some other application or EXE to run. In fact, a DLL contains various functionalities to be used by various EXE (or other DLL).

·         If DLL's are placed in some central repository then multiple programs can use the functions of DLL's at the same time. Thus saving the storage space and duplication effort. In addition, if new features are added in the new version of DLL, older version can be replaced by newer and all executables would benefit. DLL's are called dynamic links as they are loaded in the memory (RAM) when it is called by the executables at run time. They run in the program's own memory space. Though a DLL is loaded just once and whenever any other program requests to use it then operating system just maps it to the program's address space. The idea behind using DLL's is "to have a library of commonly used functions which can be obtained by everyone as they needed."

·         Microsoft has written a number of DLL's for Windows to have standard functionalities and appearances for its Operating Systems and most of the programs. As multiple applications use the DLL's, replacing older versions with newer versions of DLL's should not cause stopping running applications. System may crash or behave strangely or a program might not function, if a certain DLL is missed or the wrong version of DLL is installed in the computer.