https://msdn.microsoft.com/en-us/library/1ez7dh12.aspx

A dynamic-link library (DLL) is an executable file that acts as a shared library of functions and resources. Dynamic linking enables an executable to call functions or use resources stored in a separate file. These functions and resources can be compiled and deployed separately from the executables that use them. The operating system can load the DLL into the executable's memory space when the executable is loaded, or on demand at runtime. DLLs also make it easy to share functions and resources across executables. Multiple applications can access the contents of a single copy of a DLL in memory at the same time.

https://msdn.microsoft.com/en-us/library/windows/desktop/ms682589(v=vs.85).aspx

A dynamic-link library (DLL) is a module that contains functions and data that can be used by another module (application or DLL).

A DLL can define two kinds of functions: exported and internal. The exported functions are intended to be called by other modules, as well as from within the DLL where they are defined. Internal functions are typically intended to be called only from within the DLL where they are defined. Although a DLL can export data, its data is generally used only by its functions. However, there is nothing to prevent another module from reading or writing that address.

DLLs provide a way to modularize applications so that their functionality can be updated and reused more easily. DLLs also help reduce memory overhead when several applications use the same functionality at the same time, because although each application receives its own copy of the DLL data, the applications share the DLL code.

The Windows application programming interface (API) is implemented as a set of DLLs, so any process that uses the Windows API uses dynamic linking.