What are the components of a complete exhaust system, from the headers to the tailpipe?

 

Single would be headers, y-pipe, cat, intermediate pipe (aka "I" pipe), muffler, tailpipe(s).

Dual would be headers, collector adapters/reducers, "I" pipes, mufflers, tailpipes. I suppose you could slide cats in there somwhere if you wanted.

 

You forgot to add exhaust cutouts inbetween the headers and the cats.