When to use std::async vs std::threads?

It's not really an either-or thing - you can use futures (together with promises) with manually created std::threads. Using std::async is a convenient way to fire off a thread for some asynchronous computation and marshal the result back via a future but std::async is rather limited in the current standard. It will become more useful if the suggested extensions to incorporate some of the ideas from Microsoft's PPL are accepted.

Currently, std::async is probably best suited to handling either very long running computations or long running IO for fairly simple programs. It doesn't guarantee low overhead though (and in fact the way it is specified makes it difficult to implement with a thread pool behind the scenes), so it's not well suited for finer grained workloads. For that you either need to roll your own thread pools using std::thread or use something like Microsoft's PPL or Intel's TBB.

You can also use std::thread for 'traditional' POSIX thread style code written in a more modern and portable way.

Bartosz Milewski discusses some of the limitations of the way std::async is currently specified in his article Async Tasks in C++11: Not Quite There Yet

One simple reason I've found is the case when you want a way to detect (via polling) whether an asynchronous job is done. With std::thread, you have to manage it yourself. With std::async you can query std::future::valid() (or use std::future::wait_for/wait_until(...)) to know when it is done.