All Gmail's "folders" - even the Inbox, Sent Mail and unread messages - are actually labels, and you can narrow your email searches to them using the label: operator (or l: for short). The Google Operating System weblog lists more shorthand that refers to these special labels:
^b chats ^f sent mail ^i inbox ^k trash ^r draft ^s spam ^t starred messages ^u unread mail
Using these along with the l: operator, you can search for, say, all the unread messages not in your Inbox using -l:^i l:^u. Not exactly readable, but certainly succinct. —Gina Trapani
Shortcuts for Special Gmail Labels [Google Operating System]