Async is a library for asynchronous programming, i.e., programming where some part of the program must wait for things that happen at times determined by some external entity (like a human or another program). This includes pretty much any program that uses blocking calls (e.g., networking code, disk access), timeouts, or event loops (e.g., GUIs).
In a nutshell, the idea is to use non-preemptive user-level threads and first-class blocking operations with blocking expressed in the type system.
Read more in Chapter 18 of Real World OCaml.
Async comprises four packages,
Async_kernelcontains Async's core data structures, like
Async_js. In principle it could also be used on Windows, but no scheduler has been written for Async on Windows as of yet.
Async_unixadds Unix dependencies for things like system calls and threads. Using these, it hooks the
Async_kernelscheduler up to either `epoll` or `select`, depending on availabilty, and manages a thread pool that blocking system calls run in.
Core.Command's command-line handling
Async_rpcadds RPC support.
You can most easily understand Async's API by reading the documentation of these individually.