Managed vs. Referenced Files in Aperture Link
From the Life After Photoshop blog:
Reference library pros and cons: Your Aperture library is much smaller and can be stored on a super-fast SSD in a MacBook, for example, while your master images are stored on an external drive – and as your library gets larger, you can simply invest in larger drives. You can carry out all your organising, keywording and previewing without the masters being present, and your masters are also visible to other applications. However, there is always the possibility that your referenced files may be lost, moved or renamed and become disconnected from your Aperture library. And if you use Photo Stream your iOS images will be managed by default – so you could end up with a mix of managed and referenced files.
It’s a good intro to the concepts. Seems like poor taste to break the article over 4 pages though.