>>45A REPL is one particular Lisp program, and the traditional way of interacting with a Lisp system. It's possible to interact with Lisp systems in many other ways. It is entirely possible to have a REPL-less Lisp. However, due to Lisp programmer's fondness for REPLs, most Lisp systems do have a REPL, and other means of interaction, including mouse interaction, follow a REPL like ``protocol''.
The E in REPL stands for evaluate, and it's the step after R for read.
Evaluation may be by interpertation, or by compilation depending on the Lisp system, and your settings.
Now, the land of Lisp is vast. It's an entirely alternate universe with regards to personal computing. A good path for a beginner might be to read Practical Common Lisp and write a lot of Common Lisp programs, on your favorite UNIX or VMS clone. Later, you might try pirating or buying Open Genera, getting it working in an emulator or real Lisp machine, and seeing how it all works. However that is not a good first step, IMHO. It's best to understand how a more mundane Lisp system works and its advantages and disadvantages, before going back in time to see one approach to Lisp as an operating system.
For example, if you were to watch videos of Symbolics Genera now you probably wouldn't think it is that impressive. That's because you fail to realize things a Lisp programmer knows, that e.g. you can select any peice of text, graphic, widget etc. and then ``debug'' the object that thing represents, at run time. You might not know that there is always a way to jump to the source of the object that thing represents, modify it and recompile it and see the changes immediately etc. etc. But these luxuries you will grow to love and expect when you program in today's Lisp systems, because they are available (but not ubiquitous).
So I think learning e.g. SBCL or Allegro Common Lisp and the surrounding development tools and libraries is your best option for really
getting the significance of a Lisp OS, because then when you hear Lisp OS you won't have to see examples or anything you'll just think ``Oh my gosh! of course, I wish everything was like my Lisp''
Smalltalk on the other hand you can almost use as an operating system today (but it won't be on the bare metal like it once was), and of course it won't run your favorite applications. Just download Pharo, make it full screen and start exploring, there's two quite good free tutorial books on the Pharo system.
There are many similarities between a Smalltalk system (in the Smalltalk-80 -> Squeak -> Pharo tradition) and an Lisp sytem, especially an Interlisp system.
In other ways, Lisp systems were behind UNIX. For example, a Lisp system doesn't have any concept of ownership, so even if you can have multiple users interacting with the same machine, they can step on eachother's toes. However, most Lisp programmers will tell you that the UNIX approach to ownership is wrong, and that a hardware enforced cryptographic capabilities approach is better, and this is true. My hunch is that a hypervisor giving everyone a virtualized view of a portion of the computational resources is the best approach, and then everyone has their own personal Lisp system on top of that, but I have not given this serious thought.