When you record a GarageBand project, you use a mix of audio and musical instrument digital interface information. This collection of information is saved to a folder that contains everything your project needs to play back, but since the audio is in the uncompressed audio interchange file format, this folder is large and unwieldy to share. AIFF files require about 10 megabytes for each minute of audio at standard stereo settings, approximately 10 times the size of an MP3 file of similar length.
Combining each element of a GarageBand project into its final form is a process called mixing. This mix can include many audio AIFF files as well as any MIDI elements blended together to make a single audio file. This file can remain in AIFF format but to create manageable file sizes, GarageBand offers export to AAC format, used by iTunes, as well as the common MP3 format.
From the top menu on the GarageBand window, select "Share" and click "Export Song to Disk." You can rename the MP3 file by clicking the "Save As" box and entering a new name. Clicking "Where" permits you to choose a location for the saved file. Choose "MP3" as the export format and select audio resolution using the Quality pop-up menu. Click "Export" and your GarageBand file is created from beginning to end.
The GarageBand export function has a couple of quirks regardless of which format you use. Silence, both at the beginning and end of your project, is automatically removed, which may require adding gaps between projects if you burn a multi-project optical disc at a later date. The program uses Cycle mode to play sections of audio completely through, return to the beginning and play again continuously until you click the Play button to stop. If Cycle mode is active when you export a file, GarageBand uses only the area selected to cycle, repeating it once for the exported file.