The z-tube is the air-intake tube from a 350Z. The tube runs from the back of the box containing the air filter into the engine's intake plenum, near the back of the engine cover. The G35's intake tube contains 2 large resonators. These are chambers that catch some of the air and reduce noise at varying rpms to quiet the engine. The 350Z's intake tube has no resonators on it, is a little louder, and doesn't restrict the airflow - giving you a little bit more power.

PC is probably for popcharger, specifically the JWT PopCharger (from Jim Wolf Technolgies). This replaces the intake air box and air filter with a cone filter and heat shield. Stillen also makes an air box that's very similar to the popcharger.

One of these two intake filters combined with the z-tube is a popular combo, in lieu of a cold air intake system, and results in roughly the same net gain in HP with no worries of sucking up water in bad weather.