What is the difference between redux-thunk and redux-promise?

redux-thunk allows your action creators to return a function :

function myAction(payload){
    return function(dispatch){
        // use dispatch as you please
    }
}

redux-promise allows them to return a promise :

function myAction(payload){
    return new Promise(function(resolve, reject){
        resolve(someData); // redux-promise will dispatch someData
    });
}

Both libraries are useful if you need to dispatch action async or conditionally. redux-thunk also allows you to dispatch several times within one action creator. Whether you choose one, the other or both entirely depends on your needs/style.