Dynamic link library is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems. It is a collection of small programs that can be loaded when needed by larger programs and used at the same time. DLL files are dynamically linked with the program that uses them during program execution rather than being compiled into the main program.

In the words of Microsoft : A dynamic-link library (DLL) is a module that contains functions and data that can be used by another module (application or DLL).

DLLs provide a way to modularize applications so that their functionality can be updated and reused more easily. DLLs also help reduce memory overhead when several applications use the same functionality at the same time, because although each application receives its own copy of the DLL data, the applications share the DLL code.