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).
Read more in Chapter 18 of Real World OCaml.
API documentation for the latest release can be found here.