If signed in:
Circle = if blue, poster is not on line; if green, poster is currently on line
Triangular, red-bordered sign with exclamation point within: "Alert a Moderator to this Post" (e.g. misplaced post, personal attack, etc.)
At least that's what I see (I actually see more, but only if signed in as a moderator,) so vasantn's advice is best and easiest. Move your mouse cursor over the icon and let it hover; a dialogue bubble will show you the icon's purpose.