when the dot is green it means the person is currently on line..
when the dot is gray. it means they are offline...
the triangle thing is your ability to report a bad post... a bad post can be anything from spam (that themoderators havent seen yet, or something that you might find offensive -- just state why)
if you float your mouse over it, you shoudl get a tool tip telling youwhat it is...
the warnings are explained here:
http://www.tfproject.org/tfp/showthread.php?t=105499