SF - I don't know!
@csnyder - Can you shed some light? "Latest" lists topics in reverse chron, based on last activity. "Unread" is self-explanatory. "New" lists new topics (excludes pre-existing topics that might have new activity). What does "Top" do?
EDIT: Per the "tool tip" that pops up if you hover over the button: "The most active topics for the past year, month, week, or day." When you show the "Top" topics, you get a pull-down menu (upper right, below the Categories-New-Latest-Top navigation buttons) that lets you pick the time period you want to see. You can then click on column headers (Replies, Views) to sort the list.