Membrane technology uses a rubber "membrane" which consisting of upward indentations. These cause the resistance to the key being depressed, and have a small nub on the bottom so that the 2 metal contacts meet to complete the circuit. By simply using the rubber membrane's resistance to being distorted out of it's normal shape, resistance to a key being depressed is achieved. The resulting factor is less noise and a 'softer' feel.

Part of the Active Response membrane

Depressing one of the membranes "indentations" which are directly below the keys

The Model M has "BS" technology as was previously mentioned. "BS" stands for Buckling Spring. Each key is made up of 3 components - a key cap, an actual key, and a spring below it. Key caps allow one to reconfigure their letters without actually prying off the keys from the keyboard. This was implemented due to the aforementioned QWERTY/Dvorak controversy. The Model M is also known for its durability and weight.

The bare steel chassis and plastic [without cover or keys] weighs 1.5 LBS

The chassis is made of steel and the plastic just adds to the weight.

Other than those two factors, keyboards haven't changed a lot. The Model M uses an interchangeable socket to connect to the PS/2 port due to the fact that it was convenient to leave the PS/2 cords in and just move around with one board (as one friend of mine has done repeatedly) and due to the fact that computers still used the old AT style keyboard connector - you could use the same keyboard connected to an AT or PS/2 socket, all one needed to do was change the cord.

"Them old IBM Clunkers" as Sn1per from Virtual Hideout called them, Model M's can be had for 2 to 3 dollars at a computer show or for [usually] less than $10 USD on Ebay. They're much cheaper than the Active Responses and in my opinion, and far superior. I've even put my model M through a dishwasher to clean it - still works. Some of the new keyboards even have "drain holes." IBM anticipated people drinking in the workplace (Coffee and computer programmers are like Peanut Butter and Jelly) so they put some holes which would drain the liquid behind the keyboard. Also, most, if not all Model M's have speaker holes in the back - they are remnants from an older IBM keyboard casing. If you don't want to buy a used keyboard, http://www.pckeyboard.com/ sells brand new Buckling Spring keyboards. It should also be noted that supposedly some Pentium 4 motherboards are having troubles with the Model M& on how to modify the I/O PCB to allow the Pentium 4 motherboard to comply with the keyboard.