A Dynamic Link Library (DLL) is a library that contains code and data that can be used by more than one program at the same time. DLLs are a crucial part of the Windows operating system, allowing applications to share code and resources, reducing code duplication and memory usage. Instead of each program containing the same code, they can all access the shared code within the DLL. This also allows for easier updates and maintenance, as changes to the DLL are reflected in all applications that use it. DLLs can contain functions, classes, variables, and resources such as icons and bitmaps. They are typically used to implement common functionalities, such as printing, networking, or database access. When a program calls a function in a DLL, the operating system loads the DLL into memory (if it's not already loaded) and executes the function. DLLs can be loaded implicitly (when the program starts) or explicitly (during program execution).