What is a Camera RAW format anyway?

In simple words, a Camera RAW format is the dump of the data collected by the camera sensor, with basically no post-processing applied. While it has the pitfall of requiring heavy processing in order to achieve a displayable image, the basic advantage is that any transformatio applied to it is "reversible". From this point of view a Camera RAW format can be thought as a "digital negative". But don't confuse this with Adobe's Digital Negative format (.DNG), which is just the try to define a standard format of RAW images. Indeed every camera manufacturer (e.g. Nikon, Canon, ...) has its own format.

You can find many useful information about Camera RAW formats on these sites:

• OpenRAW
• Understanding Raw Files by Michael Reichmann
• RAW, JPEG and TIFF by Bob Atkins
• Just What is a RAW File Anyway? by Bob Johnson
• Camera raw by Wikipedia
• Looking inside your Raw image data by David Cardinal
• Reworked RAW images obtained from Russian spacecraft reveal hot Venus

Where does the name "reajent" come from?

It's a modification (where j stands for Java) of the word “reagent”:

• re·agent
• Pronunciation: rē ā′jənt
• Function: noun
• Etymology: New Latin reagent-, reagens, present participle of reagere to react — more at react
• Date: 1797

: a substance used (as in detecting or measuring a component, in preparing a product, or in developing photographs) because of its chemical or biological activity.