What are DLLs? Why do we use them?

What are DLLs? Why do we use them?
a dynamic link library (DLL) is a collection of small programs, which can be called upon when needed by the executable program (EXE) that is running.
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.