How do Clojure futures and promises differ?

Question

Both futures and promises block until they have calculated their values, so what is the difference between them?

Answer 1

There are already excellent answers so only adding the "how to use" summary:

Both

Creating promise or future returns a reference immediately. This reference blocks on @/deref until result of computation is provided by other thread.

Future

When creating future you provide a synchronous job to be done. It's executed in a thread from the dedicated unbounded pool.

Promise

You give no arguments when creating promise. The reference should be passed to other 'user' thread that will deliver the result.