Scanners typically read red-green-blue color (RGB) data from the array. This data is then processed with some proprietary algorithm to correct for different exposure conditions, and sent to the computer via the device’s input/output interface (usually USB, previous to which was SCSI or bidirectional parallel port in older units).
Color depth varies depending on the scanning array characteristics, but is usually at least 24 bits. High quality models have 36-48 bits of color depth.
Another qualifying parameter for a scanner is its resolution, measured in pixels per inch (ppi), sometimes more accurately referred to as Samples per inch (spi). Instead of using the scanner’s true optical resolution, the only meaningful parameter, manufacturers like to refer to the interpolated resolution, which is much higher thanks to software interpolation. As of 2009[update], a high-end flatbed scanner can scan up to 5400 ppi and drum scanners have an optical resolution of between 3,000 and 24,000 ppi.[3]
Manufacturers often claim interpolated resolutions as high as 19,200 ppi; but such numbers carry little meaningful value, because the number of possible interpolated pixels is unlimited and doing so does not increase the level of captured detail.
more at http://en.wikipedia.org/wiki/Photo_scanning