Sure Jazz. I think you mean in the box containing the buttons when you post?
You press the "hyperlink" button when you wish to insert a link to a webpage on the internet. You enter the address of the website between the 'URL' tags. When the post is submitted, that link will be live so to speak.
An FTP link (file transfer protocol) is used when you upload a file to a webserver and you want to provide the direct address to it. Think of FTP like windows explorer for the internet. An FTP link would show you a list of files and/or folders for you to browse through on a web-server.
Finally, the insert code button is useful when you'd like to demonstrate what a piece of code should look like without having the code actually work. For instance, if you wanted to demonstrate to somebody how they needed to put a space between their words in a post and the code for the smiley it would be very difficult for them to see how the code looks without the "insert code" button, because the forum would always display the smiley instead of the code.
Like this (This is putting a smiley inside the insert code tags):
and this is the same thing when it's not put inside the "insert code" tags:

Hope that helps.
For anybody else who is interested in exactly what all of the formatting options do or look like, we've provided a reference guide here:
http://www...on,help/page,post#bbcrefWebmaster.