DLL stands for dynamic link library. A DLL is a library that contains code and data which can be used by more than one program at the same time, with the help of a DLL; program can be modularized into separate components. For example, Microsoft Word is editing a document, the printer DLL file does not need to be loaded into RAM. If the user decides to print the document, then the Word application causes the printer DLL file to be loaded and run. Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. When these changes are isolated to a DLL, you can apply an update without needing to build or install the whole program again.
The following list describes some of the files that are implemented as DLLs in Windows operating systems
ActiveX Controls (.ocx) files
o An example of an ActiveX control is a calendar control that lets you select a date from a calendar.
Control Panel (.cpl) files
o An example of a .cpl file is an item that is located in Control Panel. Each item is a specialized DLL.
Device driver (.drv) files
o An example of a device driver is a printer driver that controls the printing to a printer.