Your browser keeps track of everything you do, and you can choose what colors will be shown for links, with different colors for "visited" and "not visited." If you use IE and set the link colors there, the same colors usually will show in Word or other Office files, although I can't say whether other browsers/programs are that closely linked.

If someone else posts to a thread that you've looked at, it's a new link, so the color changes back to the "unvisited" color. You can use that to tell if the thread has anything new since you looked last.

If you clear your browser history, lose your cookie(s), and sometimes if you do a "Disk Cleanup" your browser can "forget" where you've been and you lose the indication that you've looked at a particular link, but that's not something that happens very often. Usually the going back to the "unvisited" color just means that the target - and the link - has been changed so there'll be something new to look at if you click it again.

John