Digital backs are used in connection with photography. You are likely to have heard the words "digital camera" and the words "medium format". A digital back is a device that merges these two camera types, turning it into a superb digital camera.
Digital backs on 35mm cameras
There have been experiments on making digital backs for 35mm cameras as well, but these have largely been complete failures. The eFilm project succeeded at making a prototype, but it only worked on a few select cameras, would only have been 1.3 mega pixels, and would have cost more than a Canon EOS D60. There was once a market for 35mm digital backs, but as the prices of digital SLR cameras is plummeting because of recent technological developments, the market is vanishing. As a matter of fact, I know of several photographers who have switched from analogue medium format cameras to digital SLR cameras such as the D60 and D100.
The main problem with digital backs on 35mms is to get the synchronisation between the shutter and the imaging chip right. Because imaging chips have a very slight delay (the delay is 1/60 - 1/500 second), the shutter would have to be delayed until the imaging sensor is ready to receive information. This means that 99% of 35mm cameras - definitely all mecanical models, but also many low-to-medium end (and a few high end) cameras are ruled out, and will not be able to use digital backs. The technology behind 35mm digital backs is improving, but because the technology of digital SLRs has caught up with the old technology, I doubt we will ever see functioning digital backs for 35mm cameras.
Types of digital backs
There are three main types of digital backs:
Triple shot - These digital backs are quickly being phased out, but were traditionally the only backs available. They have a large CCD or CMOS imaging chip that can capture one single colour. A filter - either physically on the lens or inside the back, or digitally in one way or another - filters red, green and blue, and these three pictures are combined into one. Advantages of this method is that the image quality (especially the colours) can be extremely good. The disadvantage is that there has to be time between each shots, rendering the digital backs useless for anything but still shots.
Single shot - These digital backs are most similar to regular digital cameras. A large (and insanely expensive) CCD or CMOS chip captures the images. Advantages include that you can take pictures of live things (animals, people, leaves in the wind.. you get the drift), disadvantages include not as good quality as triple shot or triple sensor, but as the regular CCD technology (in consumer digital cameras) improves and prices fall, Single shot backs are used more and more
Triple sensor - This technology has never quite caught on, but was used in a few high-tech cameras back in the day. Hasselblad Dfinity, among others (not strictly a digital back, but still). This type of digital back uses three separate sensors (CCD, usually), one for each colour (red, green and blue)
A digital back can be used on many different medium format cameras - most notably from Hasselblad and Mamiya. They replace the film holder that goes in the back of the camera. Some digital backs are self-contained, with a memory chip and battery built in, but most backs need to be hooked up to a mains power supply and a computer, because of the massive data files and power consumption these devices have. This means that digital backs are mostly used in studios. The resolution of digital medium format backs have traditionally been steady on between 6-8 mega pixels, but recent development has seen imaging sensors of up to a whopping 22 mega pixels! These pictures can easily be interpolated up to any size - including your average commercial advertising board - to stunning quality that isn't a lot behind your pharmacy developing quality. Except about a hundred and fifty times bigger..
The price of digital backs is also significant. A 5-10000 USD back needs to be connected to a camera body of roughly the same price and a lens of the same price. Altogether, you end up on a rather hefty price tag.
Kodak, Fujifilm and Phase one are the most famous brands making digital backs. To be honest, they are very much a speciality item, and if you are really curious about them, you should probably go to the websites (www.phaseone.com), to find out more.
Digital backs in 2007 and beyond
Today, digital backs are still in used for high-end advertising work, but top-end SLR cameras -- like the 17 mpx Canon EOS D1s mk2 are catching up quickly. With the high-quality optics and much less hassle afforded by SLR cameras over medium format bodies with digital backs, it's widely expected that the latter will be phased out completely over the next few years.