You don't know if a question will be closed/deleted. You answer it and edit your answers because you want to provide the original poster with an answer. If you feel they'd get a better answer looking at another question, direct them to the other question. You decide what's worthwhile.
Jeff Atwood is the author of the popular programmer's blog Coding Horror. You can read more about him on his "About Me" page and on Wikipedia.
As Wikipedia says...
Together with Joel Spolsky, he has created three question-and-answer websites: Stack Overflow for programmers, Server Fault for system administrators, and Super User, for general computer-related questions.
... so he's kind of a top dog around these parts. Not only is he (kind of*) in charge because he (kind of*) literally owns the site, but he is also a community leader in that many original SO-ers were longtime Coding Horror followers.
Other site moderators will have a little
♦ character next to their usernames to denote that they are moderators. If you see a question/answer from a user with a
♦ in their username, it means they are a moderator.
As for who can override the decision of a moderator, I'm not 100% sure, and the answer would vary on the kind of decision.
In the case of closing a question, the community decides on that; closing a question isn't something only a moderator can do. Any user with 3000 rep can vote to close a question. 5 votes to close and the question is closed. Then other users with 3000 rep can vote to re-open the question and it could keep going until moderators step in.
In the case of deleting... I dunno, but I think the mods - who can see deleted questions - have a system similar to the above closing system. Don't quote me on it though.
Past this point, I now see that @Diago has answered, and it looks like his answer is overall better than mine, so I think I'll stop now.
*Use of (kind of) meant to indicate I am not a lawyer and have a complete lack of specific, accurate knowledge or understanding of the legal intricacies of ownership when it comes to the site, the questions/answers, the domain name, the blah blah blah...