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.
Async comprises four packages, Async_command, Async_kernel, Async_rpc, and Async_unix.
Async_kernel contains Async's core data structures, like Deferred. Async_kernel is portable, and so can be used in JavaScript using 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_unix adds Unix dependencies for things like system calls and threads. Using these, it hooks the Async_kernel scheduler up to either `epoll` or `select`, depending on availabilty, and manages a thread pool that blocking system calls run in.