The Apple III was introduced in 1980 and was intended as a business-oriented follow-up to the successful Apple ][.
I don't know much about it yet as I have no manuals and haven't been able to get it to do much. But it is based on the 6502 microprocessor and can emulate the Apple ][ as well as run its own operating system and software.
My unit came with 128KB of RAM, but apparently the base model came with only 96KB. Presumably this memory is bank switched, as the 6502 can only address 64KB at once, but maybe the machine uses a custom version of the 6502.
Apparently the Apple III can produce a graphics display of 280x192 pixels in 16 colours, or 560x192 pixels in 16 shades of green (on the green phosphor monitor). It produces an 80-column text display. It is also supposed to have better sound than the Apple ][.
The operating system that came with the Apple III was called S.O.S., for "Sophisticated Operating System". I haven't seen enough of it to comment on whether the name is appropriate or not. The disk format is apparently compatible to the later Apple ProDOS, though.
The above photos show the Apple III from above. The first is with the cover on. The second shows what the unit looks like with the cover off. The third is a close-up of the slots.
The top of the unit is held on with two screw-like things at the lower front corners. Turning them a half turn allows the top cover to lift off. Note that the power supply is encased in a pocket in the aluminum casing, visible to the left of the photo in the middle. My machine's power supply is quite loud, and I wonder if the sound is amplified somehow by the casing.
The four slots that are visible are compatible to the slots in the Apple ][, but the form factor isn't the same. I don't know if many boards can actually be moved between the two types of machine. The boards that were in my III are too tall to fit in my //e, and I think the //e's cards don't fit correctly in the III.
Note the reset button which is situated on the back surface of the keyboard. Also, the system speaker is visible in front of the opening for the slots.
As you can see from the pictures, the Apple III is built to look as if it has a separate keyboard, but it is attached very firmly to the unit. The front end of the motherboard is under the keyboard, and the dark bottom part of the casing is solid and extends from the front of the keyboard to the cooling fins behind the machine. You definitely can not move the keyboard onto your lap. :-)
From the side, the unit has quite an interesting shape. The front bends upward in a way that hides the feet on the monitor a little bit. Also notice that the back of the case is metal right up to the top, and is shaped as cooling fins. The Apple III has no fan, so I guess they tried to deal with heat this way. The case is very heavy and can probably soak up and radiate a lot of heat.
Looking at the back of the unit, we can see many ports along the bottom. These are labeled, from left to right, "FLOPPY DISKS", "PORT A", "PORT B" (game ports), "COLOR VIDEO", "B/W VIDEO", "AUDIO", and "RS-232C". I guess that Apple thought that four slots would be enough because the floppy controller and serial port were built-in.
I also have a ProFile hard drive unit (see above) and two ProFile controllers in my Apple III. I haven't been able to get the hard drive to work, though. It takes a long time to spin up and signal its readiness, and then when I try to access it, it sounds as if it tries to follow instructions but ends up resetting instead. If I remember correctly, the ProFile houses a 10MB hard drive (yes, that huge thing is only 10MB!), and although the drive mechanism is a Seagate the controller board which is attached to the mechanism is some custom board from Apple.
The Apple III flopped mainly because it wasn't very well manufactured. It was famous for having its chips unseat themselves, and the official remedy was to lift it up off the table and drop it. That's not very good for an expensive business computer. Some people think that, had it not been for the quality control problems, the Apple III would have been able to keep the IBM-PC at bay. I doubt it, but sometimes it's fun to play these "what if" games.