For the Microsoft Windows operating systems that are listed in the "Applies to" section, much of the functionality of the operating system is provided by dynamic link libraries (DLL). Additionally, when you run a program on one of these Windows operating systems, much of the functionality of the program may be provided by DLLs. For example, some programs may contain many different modules, and each module of the program is contained and distributed in DLLs.

The use of DLLs helps promote modularization of code, code reuse, efficient memory usage, and reduced disk space. Therefore, the operating system and the programs load faster, run faster, and take less disk space on the computer.

When a program uses a DLL, an issue that is called dependency may cause the program not to run. When a program uses a DLL, a dependency is created. If another program overwrites and breaks this dependency, the original program may not successfully run.

With the introduction of the Microsoft .NET Framework, most dependency problems have been eliminated by using assemblies.

What is a DLL?
A DLL is a library that contains code and data that can be used by more than one program at the same time. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Therefore, each program can use the functionality that is contained in this DLL to implement an Open dialog box. This helps promote code reuse and efficient memory usage.

By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster, and a module is only loaded when that functionality is requested.

Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.