Some cameras can be configured to create both a RAW and JPEG file for each shot you take, and Aperture is well suited for maintaining and working with both masters of each image. Here are some tips on using RAW+JPEG in Aperture.
When you import RAW+JPEG, the Import Panel will only show the RAW images on the memory card or other media, but the JPEG master of the images will still be imported. If you don't want both the RAW and JPEG images to be imported at the same time, simply separate them into separate folders in the Finder (drag the .jpg versions into a sub-folder, for example) before importing them into Aperture.
Under normal circumstances, you will only see the RAW master of each image in the Aperture browser, and all adjustments will be applied to the RAW images. If you would like to work with the JPEG master, highlight the image and choose Images > New Version from Master JPEG. Note that if the image you select has no associated JPEG master, this menu item will be dimmed.
In a RAW+JPEG workflow, the two images are treated as a single master. If you have imported the images as referenced, avoid deleting either the RAW or JPEG from the Finder - Aperture will lose connection with the remaining master as well.