What Kirby said, and follow the links he listed to peruse his excellent detailed explanations.
Back up originals on external drives prior to import into Aperture or any other images app. I cannot overstate how important that is, and various manuals, texts, etc. present workflows that skip that critical step.
I strongly recommend developing a rigidly followed routine like this one:
• Create a new empty folder on a hard drive. Name the folder appropriately for the camera image files that you intend to store there. Something date-based plus a name IMO is good: e.g. "20110530_Jones_Wed_mstrs." In database nomenclature 20110530 is today. In my workflow I use "110530_1238_Jones_Wed_mstrs," where 1238 is the Job Number.
In the event of multiple different projects (small "p" as differentiated from a specific Project in Aperture) on the same camera card I break out and label the different projects during this copy-to-hard-drive-folder process.
• Copy your originals from camera card to that folder then eject but do not erase the camera card.
• Back up that folder on to another hard drive.
• Review the contents of the (identical) folders on both hard drives to see that they have all the image files copied properly.
• Reformat the camera card in-camera (never in-computer) if you will immediately be using it.
In my case I have plenty of large CF cards so I store cards in a card wallet labeled "Save cards stored face down, store cards to be Erased face up." If for some reason I have not made the requisite two different-drive copysets described above the camera card gets stored face down, and is not reused until two different-drive copysets exist.
Note that there has been no usage of Aperture or any other images management program (except the Mac OS X Finder) up to this point.
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Finally we involve Aperture:
You can import each folder into Aperture as a separate Aperture Project to fully stay within Aperture's natural flow. I recommend it.
Masters are your original images, never changed by Aperture. When you make edits Aperture records the changes and Master+Edits=Versions.
An Album is just a collection of pointers to Versions, so Albums can be created and discarded at will, changing nothing and taking up negligible space. Very powerful tool. Albums are often created by searching on Key Words, another very powerful tool.
Also back up the Aperture Library using Aperture's Vaults, which are designed for that purpose.
HTH
-Allen Wicks