Assuming you're talking about the python library, it is more going for asynchronous IO and other more "concurrency" things than the data parallelism that rayon is designed for.
Hey Steve. We met at TwilioCon 2011 - not sure if you remember that. How have you been?
Is there a good guide for someone in my position? ie, to learn about tokio and rayon, having used python (for, in this case, concurrency and data science respectively)?
Oh hey! That was a very long time ago, but I loved TwilioCon. Things are good. I'm actually working on Rust full-time, so yeah, I use it a lot. :)
I'm not sure there's a great guide yet, because a lot of this stuff is still shaking out. The Rust ecosystem in general is growing at a pretty steady clip, and new stuff pops up all the time: tokio is less than a year old, for example.
There's two different kinds of problems here: "I found a library, what does it do?" and "What libraries exist?" In the former case, you're at the mercy of the library author to give you a good description. With the latter, one of the better ways is to drop by #rust on IRC, or post to users.rust-lang.org, asking for an overview of what exists. https://crates.io/search is also helpful.
In this case, rayon is for "data parallelism", meaning "I have some data, I would like to do some work on it, and I'd like to make that paralell." Tokio is about asynchronous I/O.