I've never really understood what the purpose of this thing is. Here is my guess as to what it's for (if I'm not quite right, please let me know what it's for) :

I think when we make lists, or import lists, or see a list we like in the Custom lists section, and we click on "Study List" the characters in those lists get stacked into the Queue, and they sit there until the algorithm spaced repetion thing decides that it's time for certain characters in that Queue to go there for practice. I guess the way it works is, the characters go out of the Queue in the order they were saved, or if they were practiced in a cram list and done incorectly.

Is that right?

Well, there's a review queue and a study queue... The study queue is like a study list, but more dynamic, it's an easy way to add words to study, and they add into your studies just like words from a regular list - in the order they were added, and in a cycle with the other active lists you're studying from.

The review queue, or "items due" number is the order in which items come due when you're studying them, and is based on the SRS algorithm - the better it determines you know a character, the longer the intervals between that character coming up.

how do I get to the study queue, and how do I get to the review queue ?

Your review queue (to be fair, it's not really called the queue, but plenty of users use that term colloquially for it) is the number you see as "Items due today" on the practice page. You can view the review queue to some extent under Vocabulary > Viewer, or simply look at the total items due on the practice page.

The study queue, or just, the "Queue", is the big Queue button on the Vocabulary page. You can edit it there (somewhat) like a regular vocabulary list. You can also add words to the end of the queue by using the "Add to Skritter" bookmarklet on the Vocabulary page, or pressing 'S' while an item is highlighted in the Zhongwen Chinese Popup Dictionary in Google Chrome.

thanks a lot FatDragon!!