In the context of the Ruby programming language, what is a closure and when do you use one? What are the uses for it in Rails?
Try this link: Understanding Ruby Closures. Another one: Understanding Ruby Blocks, Procs, and Lambdas. In a nutshell, a closure is a function plus a context in which the function is called. The context contains state information that the function can access, but which isn't part of the function itself. Ruby has several different kinds of closures: blocks, procs, and lambdas.
Also (from Michael): http://www.skorks.com/2010/05/closures-a-simple-explanation-using-ruby/