Axle-back systems replace everything from the rear axle back. Axle-back systems typically include the muffler, exhaust tips, and sometimes a small section of piping. For vehicles that already have good intermediate and mid-pipes, axle-back kits provide improved sound at a much lower price than cat-backs.
Typically, axle-back systems don’t improve performance much, if at all. They do replace the muffler, which can free up airflow, but the gains are minor. Since they replace such a small section of your exhaust system, installation of an axle-back kit is usually very straightforward.