Motivation and History
A long time ago, the entire telephone network was analog. This was bad, because as a voice went farther down the line, and through more switches, the quality became worse and worse as noise crept in. And there was no way to eliminate the noise, no way to know what the signal was supposed to be. Digital encoding promised a way to encode the audio such that you'd know what the signal was supposed to be. As noise crept in, you could eliminate it throught the phone network, assuming it wasn't worse than the variation between different digital encoding levels. With the transistor revolution, this theory became possible, and the phone companies began converting their own networks over to digital. Today, you have to search pretty hard to find a phone company switch that isn't digital. They call their network the Integrated Digital Network, or IDN. This solved many of the phone company's problems. However for a variety of reasons, it has been attractive to make the phone network completely digital, from end to end. For computer users, this is ideal, because we can eliminate those clumsy modems, and will hopefully benefit from higher speed. For the phone companies, they can eliminate the last of the noise and loss from the audio data. And for dreamers, this will enable a wide variety of different services to be delivered to the customer over a single interface.
What is ISDN?
ISDN stands for Integrated Services Digital Network. It is a design for a completely digital telephone/telecommunications network. It is designed to carry voice, data, images, video, everything you could ever need. It is also designed to provide a single interface (in terms of both hardware and communication protocols) for hooking up your phone, your fax machine, your computer, your videophone, your video-on-demand system (someday), and your microwave. ISDN is about what the future phone network, and information superhighway, will look like (or would have looked like). ISDN was originally envisioned as a very fast service, but this was a long time ago when it was hoped to have fiber all the way to your house. It turned out that running all that fiber would be too expensive, so they designed ISDN to run on the copper wiring that you already have. Unfortunately, that slowed things down considerably - too slow for quality video, for instance. ISDN has been very slow in coming. The standards organizations have taken their time in coming up with the standards. In fact, many people consider them to be out of date already. But on the other side of the coin, the phone companies (especially in the U.S.) have been very slow at designing products and services, or marketing them with ISDN in mind. Things are starting to pick up, but still very slowly. ISDN is available now in many places, but it is not widely used. Further most of the products and services that people have forecast for ISDN still aren't available. For this reason many people say that ISDN also stands for "It Still Does Nothing".
That brings is to B-ISDN. B-ISDN is Broadband ISDN. (The older ISDN is often called Narrowband ISDN.) This is not simply faster ISDN, or ISDN with the copper to your home finally upgraded to fiber. B-ISDN is a complete redesign. It is still capable of providing all the integrated services (voice, data, video, etc.) through a single interface just like ISDN was supposed to. But it will do it a lot faster than ISDN could. Of course, that copper to your house will still have to be replaced with fiber. But B-ISDN is still in development - it seems to be moving faster than ISDN, but it is still quite a ways off.
Fitting things together
In order to understand what ISDN is, you have to understand a bit about modern telephony. You'll invariably find lots of buzzwords, or in most cases buzz-acronyms, that seem to overlap in a terribly complex way. That's because they do overlap considerably. Nevertheless you can generalize...