What is a DLL File?

<h2>Dynamic Link Library</h2>

"A Dynamic Link Library (DLL) is a file of code containing functions that can be called from other executable code (either an application or another DLL). Programmers use DLLs to provide code that they can reuse and to parcel out distinct jobs. Unlike an executable (EXE) file, a DLL cannot be directly run. DLLs must be called from other code that is already executing."

Another way of putting it is that DLL files are like modules that can be plugged into different programs whenever a program needs the particular features that come with the DLL. The original concept behind DLL files was to simplify things. It was recognized that there were many functions common to a lot of software. For example, most programs need to create the graphical interface that appears on the screen. Instead of having to contain the code to create the interface themselves, programs call on a DLL for that function. The idea is to have a central library where everyone can obtain the commonly used functions, as they are needed. This cuts down on code, speeds things up, is more efficient, etc. They are called dynamic links because they are put to use only when a program calls on them and they are run in the program’s own memory space. More than one program can use the functions of a particular DLL at the same time.
<h2>
Examples of Important DLL files</h2>

COMDLG32.DLL
Controls the dialog boxes
GDI32.DLL
Contains numerous functions for drawing graphics, displaying text, and managing fonts
KERNEL32.DLL
Contains hundreds of functions for the management of memory and various processes
USER32.DLL
Contains numerous user interface functions. Involved in the creation of program windows and their interactions with each other

It is the common use of these types of DLLs by most programs that ensures that all applications written for Windows will have a standard appearance and behavior. This standardization was a big factor in the rise of Windows to dominance of the desktop computer. Anyone who was working with computers in the days of DOS will remember that every program had its own interface and menus.