What the hell is an editor?

[jdroa]

We talk about writers and writing and sometimes about reviewing, but...

What the hell is an editor really?

Some questions to narrow down a broad and seemingly obvious question:

• What is an editor's first duty to a writer? Why? What is their second duty?
• What is an editor's first duty to a piece? 
• How involved should an editor be?
• What should an editor focus on first? Plot? Characterization? Writing style?
• In your opinion, is an editor closer to a writer or a reader?  Or do you feel they're something else?

I'm curious to know what you guys think about editors, if you think editors are truly necessary, if you have any personal (editor) heroes, and if you consider yourself an editor before you are a writer. Also, do you have a synonymous title for an editor?  What is it?