Image Format

Digital cameras store the images they produce in two different formats: JPEG and RAW. JPEG is the most common used image format, while RAW is usually used by professionals.
The differences between the two go beyond the image size. A JPEG image is a compressed image, for example, an 8 MB RAW image can be compressed to a 3 MB full quality JPEG.
The RAW format produces much more that a BIG image. It also has been described as a "digital negative". Professional photographers use RAW because they can make later modification to the image that wouldn't be possible using JPEG.
The disadvantage is the fact that you need specialized expensive software to read and modify RAW format images. You also need time and patience. Let's say that you make 100 photos in RAW format and 100 photos in JPEG.
The JPEG images are ready to use, you may discard 20 of them because of bad exposure, but you have the rest of the images ready to be printed or to be published on the Internet.
With RAW images, you'll need at least 1-2 hours of opening the 100 images with photo editing software and tweak their properties to the point when you'll get the image you want.