I have already searched the forums and I am sure that the answers to my questions are here somewhere but I will be darned if I can find them. I have been using Garageband for a little while (version 09) but I am really confused about exactly what "plug-ins" work with this program and how to use them.

Let me explain further... I can find plenty of Mac OS X software on the internet- including here http://www.macmusic.org/software/cat.php/lang/en/id/7037/ - but I am unclear as to whether these are standalone apps, plug-ins, or something else. If they are plugins, I do not know how to differentiate whether these are intended to work with Garageband, Logic or some other software package.

Any help would be appreciated.

Audio Units are the plugin format that you want to look for. These will work in GarageBand or Logic. Check here specifically for AU plugins.

Thanks ronaldl. That's what I was looking for!