I'll let someone more knowledgeable than I answer the apertue question. Re 'RAW' files, they are files that include all information about the original photo taken by your camera before any form of change has occurred. It contains the unprocessed (hence, "raw""), pixel by pixel data, plus information, such as white balance, in the raw file header. In order to view these files, you need a program (of which there are many available for free download; a widely known one is Google's Picasa) that can load them into a viewable form using the white balance and all the other header information to set contrast, brightness, etc. These images can then be stored as any one or more of a variety of image types such as JPG or TIF. Most higher end cameras have a settable switch that allows the user to record RAW images, as well as in some other format, such as JPG. This is an option built into the camera firmware. Because RAW files have not been processed, they require the maximum amount of storage space for the particular camera taking the picture. This can easily be 10 Mb per image compared to a JPG of the same image that may be 3-4 Mb. If you take a great picture that requires no editing, the JPG type may be just as good as you can get with any conversion from the RAW form. However, if you need to edit, the fact that the RAW file contains complete, unretouched information about the original image is a major advantage, providing the greatest editing versatility possible.
There is no image type known as "RAW." Rather, there are numerous types of unprocessed files created by various types of image gathering hard/software. These are proprietary and generally such RAW images created by a device from one manufacturer cannot be read and viewed by software from another manufacturer. This is beginning to change and many different proprietary raw files can now be viewed using many of the downloadable RAW file viewers designed for generality.
The "hack" that Choyster mentioned is a downloadable piece of software that allows you to shoot RAW files on a camera that, by design, does not include the option to shoot RAW. The software sits in the directory of the memory card and, depending on the user's preferences, loads into the camera when it is turned on. The user can then save images in JPG format or in both JPG and RAW formats. For a reasonable number of shots to be stored, a 1Gb card would be minimal size, preferably 2Gb. The "hack" software has been written by programmers interested in getting more out of the hard/software built into the camera. For reasons not disclosed by Canon, for example, the capability of generating RAW files exists in most of their point-and-shoot cameras (including a570is), but has not been made available to the user by the company. The hackers have made it publically available. It supplements, but does not replace, the capabilities designed in to the cameras by Canon.
