Who are the moderators for the Ask Different site and how many are there? I've noticed many people can close a question. Are they moderators?
All users can (after they've reached a certain reputation level) participate in moderation activities on Ask Different. This is also reflected in the phrase "Ask Different is moderated by you." on the review homepage.
The Help Center lists the moderation privileges you can get; in particular, closing a question (other than your own) is reached at 3,000 reputation. Regular users can only close questions if five of them vote to do so (with the exception of gold tag badge holders who can singlehandedly close some questions as duplicates). Sometimes, people refer to regular users performing moderation tasks as 'moderators'.
That said, 'moderators' usually means '♦ moderators'; they have been elected to do moderation tasks (like closing questions) more effectively because their votes are usually binding. For more information about what we can do, see the Help Center article Who are the site moderators, and what is their role here?. There are currently seven moderators and they're listed here.
Anyone who has earned the ability to vote is on their way to site moderation and as you gain more reputation you can case close votes.
Here is the list of people that can cast close votes:
The 308 people with more than 3,000 reputation can cast close votes. Anyone with a gold badge in a question tag can cast binding close votes as can employees and elected moderators, regardless of overall rep.