There are services/applications and there are operating systems.
Operating systems run the hardware and provide an environment for software services/applications to run in.
Examples of operating systems include Windows, MacOS (formerly known as OSX), and multiple Linux based distributions (popular ones include Ubuntu, Fedora, Debian, but there are many more). Linux itself is just the core of the operating system, known as the kernel – you still need to have an operating system around that kernel, which is often known as a Linux distribution.
DietPi and Ropieee are Linux distributions that run on some ARM processor based computers, such as the Raspberry Pi.
ROCK is a Linux distribution made by the Roon Labs that runs on Intel processor based computers, specifically tuned for running on Intel’s NUC line.
Windows/MacOS were designed to run a multitude of services/applications, and thus come with added complexity to support the environment that is required to accomplish that.
DietPi has easy-to-install packages of some hand-picked services/applications, including RoonBridge.
Ropieee was designed to run RoonBridge only.
ROCK was designed to run RoonServer only.
RoonServer is the “headless” version of Roon, meaning it has no user interface. It is just the “Core” in the Roon Architecture. RoonServer can run on Windows, MacOS, or many Linux based distributions.
To interact with RoonServer, you must use a RoonRemote, which can be run on Android or Apple iOS devices. The full Roon on MacOS and Windows can also be run as a RoonRemote.
RoonBridge is an application that “extends” Roon to another device on your network, which at the moment, means giving access to all the audio outputs on that device to RoonServer. It is often used to create a network bridge for USB devices connected to another computer (but it is not limited to USB). A low power, small, and inexpensive computer for this purpose can be a Raspberry Pi.
The difference between running Ropieee and DietPi is a matter of what you find easier, and more applicable to what your needs are. The difference between ROCK and Windows/MacOS/OtherLinuxDistribution is that ROCK is optimized to do 1 thing (run RoonServer). It provides a nearly turn-key, hassle-free, stable environment to run the center of your Roon ecosystem. It is hard to mess up, but it is very opinionated on how things should work.