Downvoting and upvoting is used to indicate how "good" a question is, as defined by usefulness to the site it is posted on. Often, questions that indicate no effort spent, or that otherwise don't just make sense will be downvoted by the community. Here's the general guideline:
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
Use your downvotes whenever you see a bad question that maybe lacks details, is incomprehensible, et cetera. If a question is really bad, and not a good fit for the site, there's probably a close reason for it.
As for closing, there are several reasons defined:
exact duplicate This question covers exactly the same ground as earlier questions on this topic; its answers may be merged with
another identical question.
off topic Questions on Super User are expected to generally relate to computer software or computer hardware, within the scope
defined in the faq.
not constructive This question is not a good fit to our Q&A format. We expect answers to generally involve facts, references, or
specific expertise; this question will likely solicit opinion, debate,
arguments, polling, or extended discussion.
not a real question It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overy
broad, or rhetorical and cannot be reasonably answered in its current
too localized This question is unlikely to ever help any future visitors; it is only relevant to a small geographic area, a
specific moment in time, or an extraordinarily narrow situation that
is not generally applicable to the worldwide audience of the internet.
If a question falls under any of these categories, any user with more than 3000 reputation can vote to close it, and they are also expected to.
When there are five close votes cast, the question will be closed. These criteria generally cover questions that should not belong on the site. Therefore, they shouldn't attract answers, which is why they need to be closed. In fact, a moderator can step in and close a question right away, without needing to wait for 5 "normal" users to vote to close.
See more here:
When should I close a question?
Questions that are sufficiently off-topic, as outlined in the FAQ, should be closed by casting close votes. Questions that are sufficiently similar to older questions should be closed by casting close votes.
What's the difference?
In general, questions that get downvoted a lot also become closed quite fast, since – for example – there is a positive correlation between not showing any research effort and the "not a real question" close reason. Otherwise off-topic questions also often get downvotes, since they're not useful for the site at all.
In a way, there's a "question lifecycle" on Stack Exchange sites. After being posted, they can become downvoted, closed, and finally deleted.
If you downvote, you give the poster a chance to revise their question. Just make sure to drop a comment an tell them why you downvoted. You can then remove the downvote if the problem was solved.
If the question has more issues than just missing a bit of research, formatting, etc., and falls under any of the close reasons, it will sooner or later attract close votes. Therefore, extremely downvoted content will most probably be closed.
Closing is just the stage before deletion. Once a question is closed, it may be deleted by casting votes in a similar manner.
You see, there is a bit of a progression. The most immediate way to react on a post is by voting on it. Then, we have closing, which deals with the off-topic stuff we don't want to see or the kind of users we don't want to encourage to post more questions. Finally, there's deletion, which improves the signal-to-noise ratio even more.