Apart from a catback being less restrictive, what are the differences between the two systems?

 

well, a muffler is a part of a cat-back system. this is a term which refers to the section of the exhaust system it replaces: all of the tubing from the end of the main catalytic converter to the tips.

cat-backs are fairly ricey systems, which don't do too much to "open up" the exhaust system as much as a "header back" or "full" exhaust system, which may even include a mid-pipe to swap out the cat for those extra 1 or 2 ftlbs of torque at 7000rpm (pressure drop on modern catalytic converters is pretty small, and IMO definitly not worth taking out for 5 peak horsepower).

 

A "cat-back" system is what its name denotes. An after the cat to the back system. A muffler is part of it and is basically silences the exhaust note.

What wakeech pointed out is generally true, but not for forced induction engines.