Defining the limits of acceptable collaboration is always difficult. Sometimes professors give vague instructions, and sometimes it gets confusing when you have different standards from different professors in different classes. That's why you should never make any assumptions about whether you are allowed to collaborate. You should always assume that the answer is no, and then carefully read the assignment and syllabus, listen to what the professor says, and ask if you are unclear. Under this year's Honor Council document, it is your responsibility to know what is acceptable collaboration.
Knowing the limits is one half of the equation. The other half is giving proper credit. If you spent long hours discussing the themes from your paper on Andy Warhol with your roommate, who happened to have lived in Greenwich Village and knew people who knew him, then just cite your roommate as a source in a footnote. Then there is no doubt that you are giving proper credit for help received.
And make sure you are clear with your roommates about the kind of help you are giving them. I have seen a number of cases where a student will allow another student to read his draft or final version of paper so that the second student can get some ideas about how to get started. Sometimes the second student copies the paper verbatim; sometimes he or she uses material from it that constitutes plagiarism. Some professors will tell you that reading each other's papers is an excellent learning device, while others will say you should not do this under any circumstances. So, the bottom line again is know the limits, act within them, and give proper credit.