>If a program needs the input before continuing doesn't it need to wait

Erlang applications are developed using message passing Actors which are implemented as very light weight processes/green-threads.

So your process that sent a message can wait for the reply synchronously. It doesn't hold anything up in the overall application.

You can have hundred of thousands of these processes running in a node.

Do check it out. It is very liberating. Why it is not vastly more popular is a mystery to me.

While it's possible to have thousands of these processes running, it's completely up to the problem domain for it to be useful. If it wouldn't be bound by the problem domain you wouldn't need to write actors either.