A member recently asked this question, and it is a good one.
Whenever you add a comment to a posting on these forums, you will be given the option of doing a Fast Reply or Post Reply.
If you click on Fast Reply, a comment entry area will immediately open up on the same web page. You can then quickly enter any text reply or comment you have (including the use of smileys), and submit it. You will not have the ability to much text formatting, quoting of previous content, entering URLs, or uploading files. It is just a simple text entry form. You will probably use this 95% of the time.
However, if in your reply to a post, you want to quote a previous post, do text formatting, upload files, etc. then you should choose Post Reply. This will bring up a new window with all of these options (plus more, like spell checking).
Hope this helps answer the question.