Q:

FAQ — A guide to moderation: Voting to close a question

What does it mean to close a question? Who can do it? Why should you do it?
This is a very nice guide provided by Ninefingers on the Cryptography Meta Site. Following the example from other sites and reassured by his blessing, I'm going to post it here.

A:

Rationale: why do we close questions?
There are really two ways to do moderation. If you've been on any of the .moderated newsgroups you'll be well aware that to get a post on there, it needs to undergo a review process first and be accepted. The aim is to keep problem discussions from arising.
SE is slightly different - it works the other  way. Anyone can ask a question on SE and closing a question is the equivalent of putting that question back into review/improve mode. It's a feedback mechanism designed to react to problem cases only, so the usual business of asking and answering good questions can just... happen.
Why do we close a question?
The philosophy of SE is that each site handles questions on problems you face or things you are trying to understand. The don't ask section of the FAQ gives you a good overview of things that don't work - to summarise:

Some questions invite arguments or extended debate that suck up time when members could be helping other people.
Some questions are too broad, too narrow or have other content problems.
Some questions belong elsewhere.

These have been worked out over time and with a lot of experience from Stack Overflow and are, broadly speaking true. Sometimes, exceptions are made - it is always a case-by-case thing.
Is closing like deletion?
No. Actually, deletion is a different concept on SE. Closed questions are put "into improve mode" if you like, and are still visible for anyone to improve. That's the idea. Deleted questions are different - they have a red background and are invisible to all but high reputation users. So deletion is different and solves a slightly different problem.
What are the requirements for closing a question?
If you have 500 reputation on the site currently, or 3000 reputation on a graduated site, you can vote to close any question for the reasons above. More on that in a moment.
If you do not have this level of reputation, you will find that under the flag menu, you have an option "this question does not belong here". That will raise a flag for users who can vote to close.
Why can anyone vote to close/reopen?
Firstly, anyone can vote to close or reopen (or flag as such) because it is your site. This is really important - it's about expressing what you feel works and doesn't.
How are moderators different?
Moderator close votes complete the required vote count immediately, no matter how many people have voted. This applies for both close/reopen votes.
Initially, as the reputation requirement rules out many users, you will see moderators doing most of the closing to guide the site and look after the content. As you gain in reputation, you get to join in.
Ok, so how does it work?

Ok, the important piece. Voting to close works like this:

Underneath the question, there will be a close link. When you click on this, you will be presented with a list of options from which you can pick a close reason most appropriate to the situation.

If other people have voted, you will see blue numbers against the reason they chose.

The exact duplicate page takes you to a page that allows you to specify a question to close as an exact duplicate against.
The "off topic" page will present you with two options - one to migrate here, to meta, and one just off topic. On SE betas, only moderators can send questions elsewhere, so feel free to cast a vote here and flag if you have a target in mind.
The other options are straight out closes.
Once you pick your option and click "close", your vote is registered. You cannot retract it at this stage, but nobody else, including moderators, will know it was you who voted.
If five people agree, the question will be closed. You've seen the effect of these before.
Your name, and the name of other closers, will appear on the bottom of the question.

How does vote to re-open work?
A closed question has a "reopen" link underneath it. If you click this, a dialog box will ask you if you are sure - click yes and your vote will be registered to re-open the question.
Below, you can see an example of the re-open link with two votes registered for re-opening (out of five).

What happens if I make a mistake?
Don't worry! Unless five people agree, or a moderator agrees, the question won't be closed. 
I've seen a closed question and I disagree. What can I do?
Well, one option is to use your re-open vote! You can also always raise a discussion here on meta to seek clarification on why a question is closed and hopefully either a resolution will be reached, or you will get an explanation for the closure. 
My name appears on the bottom - isn't this going to cause problems with other users?
Actually, surprisingly few closed questions generate any response at all.
However, if you experience difficulties as a result of closing a question, you can and should contact a moderator via the flag mechanism (which is anonymous). We can then take any necessary action.
How do I find questions I might need to close? What about re-opens?
Actually there are two tools for this. 

Review. Here, there are a number of options for seeing things that might need say editing. One of these is a tab for all questions with close votes. You are then free to look at the question and decide if you agree.
Tools. Commonly referred to as the "10k tools", this page let's you see even more information - certain anonymous flags, re-open votes, delete votes etc. The important one for this is the re-open page. Here, you can see questions that might have been closed incorrectly.

