The core.async is yet another way of programming concurrently; it uses the idea of lightweight threads and channels to communicate between them.
The lightweight threads are used in languages, such as go and Erlang. They excel in being able to run thousands of threads in a single process.
What is the difference between the lightweight threads and traditional threads?
The traditional threads need to reserve memory. This also takes some time. If you want to create a couple of thousand threads, you will be using a noticeable amount of memory for each thread; asking the kernel to do that also takes time.
What is the difference with lightweight threads? To have a couple of hundred lightweight threads, you only need to create a couple of threads. There is no need to reserve memory and lightweight threads are a mere software idea.
This can be achieved with most languages and Clojure is adding first class support (without changing the language this is part of the Lisp...