I thought it might be in the board FAQ here (the FAQ link with the question mark in the balloon next to Members on the right side of the User Control Panel heading line), but didn't see it.
Here is how I think the basics work.
* Announcements have the information script "i" within a circle icon.
* Stickies have a lightbulb icon with an exclamation point in them.
* If you have ever participated in a topic, you see a red star in the upper right corner of the standard topic icon (circle with text in the middle of it.)
* If there has been activity in a topic since the last time you were online here, the icon is red. This carries through to a higher level at the Board Index level as well as the sub-forum icon will also be red if there is new activity anywhere in that forum since you last visited.
* Scrolling text within the message icon indicates a more active topic with either more posts overall or more posts within a certain time frame. I'm unsure of the specific metrics the phpBB software uses to switch from static text in the icon to scrolling text, but I'm pretty sure that's what it is meant to indicate.
Did I miss any others you were curious about?
Dude: 5' 11" - 54: Started 04/01/04
Ramblin' along in P3 since June '05...
Eat real food. Not too much. Mostly plants. -- Michael Pollan