Assuming you're not talking about values, there's really two things which can be passed to a ruby function: normal arguments and blocks.

And arguments may be passed either positionally or by name.

Blocks are the weird part, they have special support because they're a bunch of syntactic sugar / magic, which doesn't even have to be part of the function's prototype e.g

    def foo
      yield if block_given?
    end

"yield" will invoke the implicit block, "block_given?" is a metamagic kernel method which checks if the current context was passed a block.

I haven't kept up to date with Ruby so I don't know if that style is still en vogue. The equivalent "explicit" version is

    def foo &block
        block.call if block
    end

in which the block — if passed — is reified to a Proc, which you can manipulate normally. If no block is used, then `block` will just be `nil` (as the essay notes, blocks are always optional).