The bit-depth described the amount of information the computer is using to represent the range of colour, or tones in black and white, present in an image. This can either be 8 bit or 16 bit. 16 bit scans are also called 48 bit scans because 16 bits are used to describe each of the three colour channels.
16 bit scans were invented, using 16 bits of information to describe each colour, or giving a tonal range of several billion colours
8 bits implies that each of the three colours in light (Red, Green, Blue or RGB) are represented by 8 bits of data. In practice this means each of the RGB colours is represented as one of 256 tones, or through combination that your image is represented by some 16 million tones. This is far in excess of the colours/tones available when printing (even on the very best printers), and so 8 bit files were for a long time the industry standard colour depth used for images for printing.
However, as the industry matured, Photoshopping images became the norm. Sometimes images were extensively manipulated. This resulted in problems with 8 bit scans, as effects such as 'posterisation' would occur. Essentially this means that when Photoshop is performing its very effective trickery, it would run out of information, and the range of tones in the image would be 'clipped' and the image would no longer contain smooth tonal transitions. This resulted in the classic early digital look of colour banding. Hence, 16 bit scans were invented, using 16 bits of information to describe each colour, or giving a tonal range of several billion colours (65335 levels per channel).
At Image Science we only scan in 16 bit so you can be sure you're getting the best quality result.