1. It's a Ruby implementation.
2. It's mostly compatible w/ Ruby & its components (Rails, gems, etc.) but not completely.
3. ... ?
An About page would be nice - answer these questions, among others:
1. What is Rubinious?
2. What differentiates it from Ruby?
3. What problems does it solve?
4. Why would Ruby programmers want to use it?
5. Why would non-Ruby programmers want to use it?
From http://rubini.us you can read "An environment for Ruby, the programming language that provides performance balanced with accessibility, focusing on improving programming productivity"
The thing is, in this context, what is "an environment for ruby"?
Rubinius is a ruby interpreter written in C++ using many advanced techniques and technologies such as LLVM, a "precise, compacting, generational garbage collector", a compatible C extension api to standard MRI Ruby and many other awesome ideas that will give any language nut a huge nerdon.
From http://rubini.us :
Wherever possible Rubinius is written in Ruby. Where not possible (yet), it's C++.
And further down:
The Rubinius bytecode virtual machine is written in C++, incorporating LLVM to compile bytecode to machine code at runtime. The bytecode compiler and vast majority of the core classes are written in pure Ruby.
So, it's the VM that's C++. The bytecode compiler (which is technically what's meant by "interpreter" in this case) is ruby.