What is the difference between enum and shared immutable?

[Jan Hönig]

Maybe this is a silly question, but I don't understand completely 
the difference between an `enum` and a `shared immutable`.

I could have:

enum x = 1;

or

shared immutable x = 1;

What is the exact difference between those too?

My best guess is, that shared immutable gets initialized during 
the beginning of the programm execution.
Enum has to be "ready" at compile time.
Is this this accessment correct and complete?
Am I missing anything else?

(If correct, then I understand that shared immutable is capable 
of doing some sideeffects during is construction (if the 
constructor does that))