A DLL (Dynamic Link Library) is a type of shared library file primarily used by Microsoft Windows operating systems and applications. It contains code, data, or resources (like icons or dialog layouts) that multiple programs can use simultaneously. Unlike including code directly into an application's executable file (static linking), DLLs are separate files linked at runtime. This means different programs load the same DLL copy from storage into memory when needed, rather than each program carrying its own duplicate copy of the functions within it.

These files are essential building blocks. For example, a company might develop a single security authentication .dll file that all their different Windows applications use to verify users. Similarly, a graphics application might rely on a rendering .dll provided by a graphics card manufacturer to handle display functions efficiently. Software plugins (like custom filters for an image editor) are also often packaged and distributed as DLLs. The Windows operating system itself heavily relies on core system DLLs (like kernel32.dll) for fundamental operations.

The major advantage is code reuse and efficiency: updates to a single DLL can benefit all programs using it, executables are smaller, and memory usage is optimized. Key limitations include the "DLL Hell" problem, where conflicts between different versions of the same DLL cause program crashes. Security risks exist, as malware can sometimes inject harmful code into trusted DLLs or replace them. Modern Windows versions mitigate issues using mechanisms like Side-by-Side assemblies and digitally signed DLLs, though careful management remains important. Containerization technologies also influence how libraries are deployed and isolated.