A DLL, or Dynamic Link Library, is a type of file used in Microsoft Windows operating systems that contains code, data, and resources shared by multiple programs. Instead of each program having its own copy of common functions, DLLs allow these functions to be stored in a single file and accessed as needed, saving memory and disk space. While DLLs improve efficiency and simplify updates, they can also be exploited by cybercriminals. Malicious DLLs, if loaded by a program, may give attackers unauthorized control over a system, making DLL security an important aspect of cybersecurity.