Blank = No Triager has worked on the bug.
Need User Input = Triager has asked for more information, is currently waiting on the reporter to reply
Needs Developer Input = Triager Or Reporter has asked for a dev to assist
Needs Stacktrace = The bug is a crash/hang that needs a stacktrace
Needs Testcase = Bug needs a testcase that illustrates or causes the issue
Needs Other Input = Bug needs input other than the above, reference the comments and whiteboard
Once the bug is fully triaged, it will be moved from the Untriaged product into the specific product it is, and these flags will no longer be visible.
This list draws heavily from Tmyoung's proposal at User:tmyoung/Bugzilla Triage.