Basically it's a terminal all of whose entire operations are controlled from the server.
Bootp, now DHCP, was originally developed to allow diskless devices to boot an OS from a network server. Thus Sun's ELC diskless workstation, circa 1988, ran a full SunOS implementation locally, but loaded it from a central server. Since disk was expensive at the time this looked like a great way to save money but was never really successful because it generally seemed to combine the worst of both worlds: having a general purpose local OS and having to load and save over a relatively slow network.
The distinguishing characteristic of these things is no longer the network OS boot - since some now boot Linux or Windows CE from ROM or a local disk - but the ability to concurrently show applications running locally and applications running on a server.
Today there are at least a dozen companies making these and they all have exactly the same problem: they're cheaper Windows boxes with a different brand name and administrative structure. They're not useless, of course, but they're not a general purpose solution either.
The Linux terminal server project, for example, falls into this category - and I just recommended this solution to an ask bloggie volunteer who needed a way to make use of some older PC gear.
The first real smart displays, NCD's X-terminals also booted an OS from the server - but one limited to running the display. As a result the late eighties and early ninties models, things like the 19C and HMX series, were almost 100% reliable - in fact I bought a a batch of 30 used ones with 21 inch screens in 1990 and not one of them failed during ten years of nearly continuous power-on.
The next generation did move toward running some applications on the terminal CPU - this Explora Pro, for example, had the ability to run Mozilla from ROM and marked the beginning of the end for NCD. I mention this one, however, because the Explora was the only series with which, having run hundreds of the things for years, I've ever had any problem -and the story is worth sharing for the laugh value.
See that little black box, about the size of a thin paperback? That's where the processor and memory are. So I had one user whose Explora kept mysteriously failing - he'd be working away when the screen would fade, colors would change, or the picture would distort. he could show it to me after it happened, but no matter how long I sat in his office it would never happen while I was there, and switching the boxes produced the odd effect that the defective one would work perfectly, and the good one, now in his office, would start to fail.
Finally, of course, I caught him - he was putting the processor box on the radiator, dropping a sweater over it, and then putting it back on his desk while calling me. I never did understand why he did it, but this was the only failure incident (other than an office manager who dropped a monitor after discovering it weighed 85 pounds - and a secretary who shipped it by Fed Ex overnight for replacement!) I ever had to deal with.
Sun's Sun Ray carries this evolution one step further. The Sun Ray is not an X-11 device, it literally just the information display and input hardware for an X-client that runs on the server. That's why you can take a baseball bat to one while someone's using it, and have that person continue typing on another one with no more hassle than signing in again.
One good way to see what the difference amounts to is to think about in terms of a voting machine for use in elections. Put any kind of local OS on the machine registering the vote - whether that's a PC or Winterm doesn't make any difference - and you cannot ever hope to prove the device secure against tampering. No matter what fancy technologies or processes you invent, any real expert hauled into court to testify about the possibility of vote tampering at source would have say "NO" when asked if tamper is impossible.
In contrast, set this up using a Sun Ray client, and those experts, facing the same question, would be saying "Yes" -because tampering at source is proveably impossible if there's nothing there to tamper with.
So what's a smart display: in the hardware sense it's an information display and inut device - nothing more. Not a client, not a thin client, not even an ultra-thin client - it collects keystrokes, mouse movements, whatever - and displays the server's response. That's what NCD's early X-terminals did, that's what Sun Rays do - and the power comes from the simplicity: it does one thing, and does it very well.