Homework questions are definitely allowed on this site!! In fact, the major purpose of the site is to provide a place to discuss homework and other problems coming from class (like on review sheets) in detail. However, a good question will state the problem completely, using mathematical notation, and indicate any progress that the you've already made.
Example HW questions
Not so good
How do I do problem 1.3.4 on page 18?
A better question - we can improve the question by simply stating the problem.
Problem 1.3.4 on page 18 of our text states: Assume that $A$ and $B$ are nonempty, bounded above and satisfy $B\subseteq A$. Show that $\sup B \leq \sup A$.
I will admit that I am clueless on this problem - any suggestions?
Even better - Ideally we've made some progress on the problem, which we can state.
Problem 1.3.4 on page 18 of our text states: Assume that $A$ and $B$ are nonempty, bounded above and satisfy $B\subseteq A$. Show that $\sup B \leq \sup A$. I can see that an upper bound of $A$ must be an upper bound of $B$, but I don't quite see how to polish off the problem.
Other types of questions
Of course, other questions related to course material are welcome as well - but they must be directly related to our course and be specific, answerable questions. For example, problems out of the text but not directly assigned or from other source, but clearly related to our course, should be just fine. Open ended questions like "What's all this stuff good for, anyway?" are not a particularly good fit.
Questions about the operation of this site itself are also good; they should generally be tagged Meta.