*.dll files are archive files open with winzip/7zip etc. That isnt to say that all .dll files are archives you can save anything with the .dll extension however most windows .dll files are generated to be archives examples of this are windows>twain_32.dll which is an archive file however twain.dll is not if you look at twain.dll you will see an MZŽ as the first three notepad characters which denotes a Compiled C file/program or part of a program. Whereas MZ seems to be an archive.
Also most .exe files are archives mostly containing an icon image etc for the file and the windows installer packages as well they contain all the information the program needs to run images,movies etc and also directories including installation information and plain text file.
I have a game here game.exe and it contains java class files an image a pointer directing the .exe to run a .bat file. Obviously your .bat file will run a javac call from the archive and run the game there is also a few .dll archives containing java class files also.
[autorun]
ICON=AUTORUN\MINCRAFTLOGO.ICO
standard icon redirect here .ico is an image file within a .dll file within a .exe file. So the image seen on the .exe file is the minecraft logo. This is in a file called autorun.inf. Second example
[discstarter]
startpage=Autostart\Disk1.html
uselanguagestartpage=1
windowcaption=Solid Edge
licensee=Siemens PLM Software
productguid=05B227DF-DB00-4934-B3C8-40B7D8FAA54A
singleinstance=1
hidesplashscreen=1
noscrollbars=0
showstatusbar=1
splashscreentime=0
windowwidth=750
windowheight=775
buttondir=Autostart
toolbarcolor=16777215
toolbar=goback,goforward,gohome,print,exit
[autorun]
open=autostart.exe
icon=Autostart\ENGINE.ICO
This is the solid edge autorun.inf file contained in solidedge.exe Autostart\ is the Autostart.dll directory. open=autostart.exe specifies the autostart.exe file to run from within the original solidedge.exe archive. Here is a sample program using the .dll (dynamic link library) files http://www.flipcode.com/archives/Creating_And_Using_DLLs.shtml.
It also shows how they are created. As you can see the contents of the dll file is called by an exe file as I previously explained also there is a tutorial here http://msdn.microsoft.com/en-us/library/ms235636.aspx and as i said before 7zip or winzip will open a dynamic link library as an archive as long as you have the .dll file. If the contents of the dynamic link library have been compiled obviously you need a program which can read the file.
However since .dll files are by definition just archive library files the dll itself should be readable and not a compiled C,C# file etc etc Basically .dll files are archives well should be when a .dll file is created in visual studio the dll is created and any information you store in the dll file is encrypted. Mostly this encryption is handled by visual studio itself and generally isn't edited by hand. When you read a .dll file contents as a .exe the contents are automatically decrypted. Now when we talk about compiling a program we are changing the contents into bytecode the machine easily interprets.
This filesize would be smaller than the original file of the same contents. However the filesize is larger suggesting that the file has actually been encrypted. Probably to stop people reading their code. As a result the reading of .dll contents is termed decryption and not decompilation. Decompilation would convert the already compiled txt files to unreadable byte code. The use of standard .dll files is by definition not opensource because it involves the deliberate obfuscation of byte code.