DLL-Dynamic Link Library, a library linked to an executable program at run-time. It refers to a file with a .dll extension. A DLL is a set of functions that can be executed, or data that can be used by a Windows application. Several different programs reuse the same DLL instead of having that code in their own file, this dramatically reduces required storage space. DLL (Dynamic Link Library) files are basically "support" files for certain types of software. They are generally (but not always) found in your Windows System directory

Advantages of Dynamic Linking
The advantage of DLL files is that, because they do not get loaded into random access memory (RAM) together with the main program, space is saved in RAM. When and if a DLL file is called, then it is loaded. For example, you are editing a Microsoft Word document, the printer DLL file does not need to be loaded into RAM. If you decide to print the document, then the printer DLL file is loaded and run.

• Multiple processes that load the same DLL at the same base address share a single copy of the DLL in physical memory. Doing this saves system memory and reduces swapping.

• When the functions in a DLL change, the applications that use them do not need to be recompiled or relinked as long as the function arguments, calling conventions, and return values do not change. In contrast, statically linked object code requires that the application be relinked when the functions change.

• A DLL can provide after-market support. For example, a display driver DLL can be modified to support a display that was not available when the application was initially shipped.

• Programs written in different programming languages can call the same DLL function as long as the programs follow the same calling convention that the function uses. The calling convention (such as C, Pascal, or standard call) controls the order in which the calling function must push the arguments onto the stack, whether the function or the calling function is responsible for cleaning up the stack, and whether any arguments are passed in registers. For more information, see the documentation included with your compiler.

All in all a DLL is an executable file that cannot run on its own, it can only run from inside an executable file.A potential disadvantage to using DLLs is that the application is not self-contained; it depends on the existence of a separate DLL module. The system terminates processes using load-time dynamic linking if they require a DLL that is not found at process startup and gives an error message to the user. The system does not terminate a process using run-time dynamic linking in this situation, but functions exported by the missing DLL are not available to the program.