Every message posted to each section of the forum board are listed by the most recent posted message first. This does not include the announcements at the top of each forum or some posts that are permanent at the top of the list as well.
For example, under the Ride Board, we have permanent announcements at the very top. There is a division line as you scroll down to separate these from the request board. Next there may be a few posts that always stay at the beginning of the request section of the board. These are often times a pilot offer who might fly a particular route frequently. Then the regular daily request posts will start and move down as each new request is posted. The most recent are always at the beginning.
You can also log in and on the board index page click on "view new posts". These are posts that you have not read yet. Or you can click on unread posts and those will come up for you as well as posted you have not seen. The list is normally long if you do that though.
Hopefully this makes sense for you.