Karen,
now you know HOW to do it, make sure you understand WHY you are doing it.
GarageBand is a content-creation app like Logic, Final Cut Pro X, Photoshop and even Word. These are apps that lets you create new content in the form of new audio files, new videos, new images or a new book. While in the process of creating that new content which can take hours, month or years, you have to save that work (work in progress). That work is saved by the app in its own proprietary file format. This is often called a "project file"
Project File
A project file from a specific app can only be opened by that app (or an app that is compatible with it). The purpose of that file is to work on the content until you finalized it (the new song, video, graphics or book). In GarageBand this project file is the one with the .band file extension that is created when you use the save command in GarageBand. You save your Song as a GarageBand Project File. Although it s also called a "Song File", this term can be misleading, implying that it is an audio file like the ones in your iTunes playlist.
Audio File (video file, graphics file, text file)
Once you are done with your creation, you have to save it to a file format that can be opened/played by apps other than the one you created the content with (for example, iTunes cannot play your .band GarageBand Project File). This common step has many different words: "save as", "export", "share", "render", "bounce" etc. You could use the term "convert" but it might be misleading because it is more used for the process of changing an existing audio file (or vide, graphics file) from one format to another (i.e. aiff>mp3, wav>AAC, tiff>jpeg, etc)
Convert
There are many different apps that lets you convert files from one format to another. Leoni showed already how to iTunes with it. This conversion step is necessary if a content-creation app doesn't provide the option to "export" to that specific file format. For example, GarageBand cannot export to a wav file. SO you have to export to an aiff file and then do the extra step to "convert that aiff file to an wav file.
Once you wrap your head around it, it is actually a simple concept and with the right terminology makes a lot of sense.
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'