confused about DLL

Question

I've tried to read about DLL on the web, but I'm still confused about the whole concept of them. So from what I understand, they're used to share codes between one to many programs? So what's the difference between DLL and actual code files?

So if I wrote a code in C#, I could save it as a DLL and share that among other programs? How is a DLL different than a EXE?

Any help would be appreciated, thank you.