The document discusses Dynamic Link Libraries (DLLs) in Windows. It explains that DLLs contain functions that can be called by other executable files or DLLs. The three main DLLs in Windows are Kernel32.dll for system functions, User32.dll for user interface tasks, and GDI32.dll for graphics functions. Other DLLs provide additional functionality like object security, common dialog boxes, and window controls. DLLs allow code and data to be shared among applications to improve efficiency.