Short for dynamic-link library, a DLL is a common library used in Microsoft Windows platforms. They are essentially a pool of resources that can be used by many programs. Files within a DLL are designated with the .dll file extension. A DLL file is used to perform a specific set of tasks. For example, it may provide the steps necessary to print a document. Windows puts all the DLL files in the same place to make it easier for developers to write programs. Without DLL, developers would have to write all the steps themselves and always be re-inventing the wheel.