RAW files are the actual data taken directly from a digital camera’s image sensor. They have not been processed by the camera at all. This means they are the purest image file possible in digital photography. Often, quite correctly, the terminology “digital negative” is used to describe them.
Different camera manufacturers have different names for their own RAW files. For example, Canon uses the digital photography terminology “RAW”, Nikon uses different digital photography terminology – “NEF”. They are incompatible with each other. Whereas every computer can show JPEG images, RAW files need specific software. This is always supplied by the manufacturer when you purchase a digital camera. It is also easy to obtain on the internet too.
Glossary of Digital Photography Terminology – http://www.digital-photography-tips.net