Let's take a look at the ZDefocus node. This is the node you want to use with elements that come with a depth pass, like CG renders. It's gives you a lot on control to make beautiful and realistic out-of-focus images.
The copy node is somewhat similar to shuffle and shuffleCopy but with a different interface and concept. The shortcut for the copy node is K. Just like a merge, it's meant to be use "A over B" so you copy elements from the A pipe to the B pipe. In the interface, the left part is the channels from the A pipe and the right part is the B pipe, or output.
Understand and learn how to use the shuffle node in Nuke in 4 minutes. This quick tutorial will teach you what you need to know so you'll be able to shuffle channels like alpha and depth into RGB. Also watch the next video about the shufflecopy to understand the difference between the two.