RISC OS is an operating system for ARM microprocessors. The acronym RISC OS stands for Reduced Instruction Set Computer Operating System, which makes sense as ARM microprocessors traditionally have a very strict RISC architecture.
RISC OS was developed in 1989 by the British computer company Acorn Computers Ltd. (which is no longer in business). It was originally written for Acorn's Archimedes home computer series and was the successor of the Arthur 1.2 operating system.
Today, RISC OS is owned by Pace Micro Technology and is maintained by RISCOS Ltd. (based in Cardiff, UK) and Castle Technology Ltd. (based in Framlingham, UK).

I am RISC OS user since 1990, when I bought my Archimedes A420/1, which had RISC OS 2.0 installed.
For a while I was even professionally developing software for RISC OS. Back in 1994 I developed the Arranger and Precision tools for the ArtWorks illustration program.

So, what does RISC OS have that other operating systems don't have? Interestingly, this question actually has to be answered with "not much", at least from today's perspective.
The main reason for the popularity of RISC OS is probably its very clever graphical user interface, named "Wimp". RISC OS uses context-sensitive pop-up menus, as opposed to pull-down menus (used, for example, by Windows and Mac OS). The middle mouse button will always display the appropriate menu for the current location of the mouse pointer; no moving of the mouse to the menu bar at the top of the screen is necessary, all functionality is instantly accessible. Also, RISC OS implements Drag & Drop in a very consistent way. All types of data can be transferred between applications with a simple mouse drag. File selector dialogs are not necessary because files can be dragged directly from/to a directory. From the very beginning, RISC OS provided a font manager with fully anti-aliased outline fonts.
Of course, all these things are no longer a unique features today. But who could imagine all this way back in 1989? Well, for RISC OS users these features were already a common thing in 1989; the basic way of working with RISC OS has not changed since then. Still today, RISC OS solves many problems in a more elegant and efficient way than most other graphical user interfaces.
Another unique quality of RISC OS is its modularity, performance and compactness. The whole operating system is very slim and most of the processor performance actually reaches the user and is not consumed by a heavyweight OS. Browsing the Web on a 200 MHz Pentium running Windows is probably not much fun these days (and probably requires that the system has a least 64 MB of RAM). On the other hand, a 200 MHz StrongARM system with RISC OS still today provides ample performance to view and navigate the web smoothly, and it will also run with only 16 MB of RAM.
RISC OS itself is usually located in a 4 MB ROM or can be softloaded. It has no virtual memory system, but still most of the time more than half of the RAM will not be in use on a 64 MB system under typical use. This is due to its module concept, which allows applications to share common code. Applications that work with large amounts of data usually come with their own optimized memory management system (and do a much better job than a generic virtual memory manager, which is forced to rely on heuristics and has no knowledge about the structure of the data).

If any of that sounds interesting to you, check out the links below! RISC OS is still around! It is still being maintained and further developed!

RISC OS is available in two different variations:
RISC OS Select is based on RISC OS 4 and will run on systems with older ARM processors (ARM610, SA-110) that still support the 26-bit mode (for example, the Acorn RiscPC).
RISC OS 5 is a separate development and works exclusively in the 32-bit mode of the ARM. It runs on more modern ARM-based processors like the SA-1110 or the Intel XScale (the Iyonix PC is an XScale-based system that runs RISC OS 5).