Dynamic Link Library also abbreviated dll is how the shared library concept is implemented in the window operating system. DLL files are collections of small programs that can be called upon by a larger program being run on a computer. This is accomplished via either a static link or a dynamic link. Static links are used to permanently connect a computer application to a DLL file. These are used when a computer application is using the information contained in a DLL file repeatedly or often. Dynamic links allow a program to access DLL file information only when necessary. Therefore, the links are only created when the application is using the information in the DLL files. Dynamic links to the library are created and closed on an ongoing basis while the application is running. Because these links are rapidly changing based on the needs of the applications, they are called dynamic links.

The shared libraries usually have .dll as a file extension although they have the same file format as windows EXE files. Dynamic Link Library (.dll) files contain a combination of code, data, and resources that are vital for the proper operation of your computer and its programs. The original intent of dynamic link libraries was an effort to conserve both memory and disk space neccessary for program applications by having it stored locally in the hard drive. If two parts of two programs use the same code, rather than include that code in both programs, it is instead written to a dynamic link library or dll file where it only has to be loaded once by the computer and both applications can share the code and other information contained in the .dll files.

Because the DLL files are not loaded into the random access memory of the computer with the applications, the computer programs are able to run on far less RAM than if the DLL file information was included when the program is loaded. Because of this, computer programs are able to run faster and more applications are able to be run at the same time on a computer. The use of these dynamic link library files not only allows windows and other operating systems to run on less memory than would otherwise be possible, but the space required to store a program on a computer is also reduced by allowing multiple programs to share code and other information through the use of DLL files.