The images in digital cameras are simply a collection of dots just like the dots that form an image in a newspaper or magazine.  Each dot is called a pixel and the pixels have a color depth that ranges from black and white to true color.  The lowest color depth is represented by a byte (8 bits) per pixel which results in 256 colors.  High color is represented by 2 bytes (16 bits) which results in 65,536 colors.  True color is represented by 3 bytes (24 bits) which result in 16,777,216 colors.

The size or resolution of a photo refers to the number of pixels used in the photo.  An good size for emailing is 320 pixels wide by 240 pixels high (approximately 4" x 3") for a total of 76,800 pixels.  Using 24 bits color depth (3 bytes per pixel), this results in a photo of 230,400 bytes or 225 KB raw data.  Similarly, a high resolution (2048 x 1536 pixels - 3145728 pixels or 3 Mega Pixels) photo suitable for making 8x10 photos will result in a 9 MB photo of raw data.  However, digital photos are usually stored as JPEG files which (depending upon the amount of compression) can compress the raw data 7.5 times to result in a file that is only 1.2 MB.