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Every image you shoot is stored on your camera’s media card as an individual file. The file is a document just like you might create on your computer. Importing (some people call this step ingesting, but that sounds a little too biological for me) is simply the process of copying those image files from your camera’s media card to your computer. To import, you can connect your camera to your computer via a cable (usually USB-2) or take the media card out of your camera and put it into a media card reader attached to your computer.
Depending on the type of reader, a media card reader can be faster than plugging your camera into your computer, and it doesn’t drain your camera battery. Also, most card readers support lots of different formats, so if you have more than one camera (say, an SLR and a small point-and-shoot) that use different formats, then you need to carry only one card reader and a cable.