01In plain English
A raw file keeps everything the camera sensor captured, before any sharpening, colour or compression is applied. It has to be developed in a raw editor, where you set exposure, white balance and colour yourself.
02Why it matters when you are choosing
Raw files let you recover blown highlights and dark shadows and fix white balance without losing quality. If you shoot raw, a raw editor matters more than any other feature.
03What to check
Ask the vendor
Check that the editor supports your camera model, and whether it also organises a large photo library or only edits one image at a time.