What is "dll"

Dynamic-link library (dynamic link library) or DLL, is the implementation made by Microsoft for the concept of shared libraries in Microsoft Windows and OS/2 operating systems. These libraries usually have DLL, OCX (for libraries that contain Activex controls), or DRV extensions (for legacy system drivers).

The file formats for DLL are the same as the executable files for Windows. Like executables (EXE), DLL’s can contain codes, data, and resources (icons, fonts, cursors, among others) in any combination.

In the broad sense of the term, any data file with that same format can be called a resource DLL. Examples of these DLL include icon libraries, which may have the ICL extension, and font files, which have the FON and FOT extensions.

Source: DLL - Wikipedia