The format is [a href = http://www.habitablezone.com/currentevents/messages/46211.html] this is a llink to your post [/a]
and replace the [ ] brackets with < > ones. You can put any text in the middle describing the link, most people keep it fairly short. It's nice to say where the link is from too. Folks here can be sticklers about attribution.
For some reason if you put a link as the very first item in your post, you end up with a < showing - the link still works but looks a bit untidy.
You can link images directly to your post with this format:
[img src = 
Again replace [ with <. No description needed.
Note that some readers may have slow internet connections and so pictures may bog them down - it's nice to warn if you embed a picture in your post by saying something in the subject line.
Current Events » in reply to Arnette In The Unemployment Line
Links are simple