What is a 'Closure'?

Question

I asked a question about Currying and closures were mentioned. What is a closure? How does it relate to currying?

Answer 1

Kyle's answer is pretty good. I think the only additional clarification is that the closure is basically a snapshot of the stack at the point that the lambda function is created. Then when the function is re-executed the stack is restored to that state before executing the function. Thus as Kyle mentions, that hidden value (count) is available when the lambda function executes.