You can save both Real and Software Instrument regions you record as Apple Loops. When you save a region as an Apple Loop, it is added to the loop library and appears in the loop browser, so you can use it in other songs.

Apple Loops you create from recorded regions match the tempo and key of any song you add them to, just like the Apple Loops included with GarageBand.

1. Select the region in the timeline.
2. Choose Edit > Add To Loop Library, or drag the region over the loop browser.
3. In the Add Loop dialog:

   Type a name for the loop.

   Choose the scale and genre from the pop-up menus.

   Choose an instrument category from the list on the left.

   Choose an instrument name from the list on the right.

   Click the appropriate mood buttons to add mood descriptors for easy searching.

4. Click Create.

The loop is added to the loop browser, and becomes blue to indicate that it is an Apple Loop. You can find and audition it using the keyword buttons, menus, or by typing the name in the Search field. .
There are also very powerful sound and loop editing programs available like Ableton Live , Propellerhead ReCycle , BIAS Peak and Sonic Foundry Acid that can create a huge variety of original sounds to be used as loops. It doesn't matter in what format you import them: GarageBand will convert them in a couple of clicks using the above method

• You can import a Midi file by dragging it to the main GarageBand window, where it will show up as a green track.
• Cut it down to the section you want to use as a loop
• Play with the notes in the region: change pitch, velocities, rhythm as much as you like. This way you can truly make the loops your own, without having to start from scratch.
• Tip: Use notation view for editing a software instrument track. Even if your reading is not too hot, it will soon make sense and you are learning a skill that will prove very useful over time
• Now save as an AppleLoop

Option-drag a Software Instrument loop to the timeline, to the empty area below the existing tracks.

By default, option-dragging a Software Instrument loop to the timeline converts the loop to a Real Instrument loop. You can change this behavior by choosing GarageBand > Preferences, clicking the Advanced tab, then turning off the "Convert to Real Instrument" checkbox under "Adding Loops to the Timeline."

• Real Instrument tracks, containing pre-recorded audio data
• Software Instrument tracks, containing midi data
• Recorded Instrument tracks, containing your recorded audio data.
  

  All can be converted to loops: the audio data become audio loops (blue icon), the software instrument data become midi loops (green icon)
  
  

• The advantage of midi loops is that you can change the sound, and the rhythm, pitch and velocities of individual notes. For a detailed look at how to do this creatively, see the GarageDoor: Changing Loops to exotic Scales
• The advantage of audio loops is that they are easier for GarageBand to handle, and will be much less of a drain on your Mac processor.