Two general points
- Red icons are unread items, blue/grey are read
- A star on any topic means you have posted in it

- Announcement

- Sticky topic

- Read topic

- Unread topic

- Read topic that you have posted in

- Unread topic that you have posted in

- Hot topic

- Locked topic

- Moved topic

- Forum with subforums (which have been read)

- Forum with subforums (which are unread)

- Link forum

- Attachment

- Go to last post in the topic

- Go to last unread post in the topic

- Subforum read

- Subforum unread
Source:
http://www.phpbb.com/community/viewtopi ... 4&t=587802