Those are the basics. The gear set is referring to the ring and pinion gears in your rear end, and the ratio at which they turn in relation to the driveshaft. The lower the gears (higher numerically), the quicker the rear tires spin, and the faster the car accelerates.
The posi reference is also regarding the rear end. A posi rear end is one that has a limited slip, meaning that as soon as the primarly driven wheel starts to spin (slip) , without the vehicle moving forward (smoking the tire), a series of clutch discs are centrifugally thrown together grabbing the non moving axle, and forcing it to spin too. The idea is that as soon as one tire starts to lose traction, the other becomes driven to help move the vehicle. Most stock rear ends (other than GT's) are only "one leggers", meaning that no matter what happens, only one axle will be driven.
The cold air intake simply locates an open element air filter in place of the air silencer inside the passenger side fender. This allows the air that goes into the engine to have less restriction, and to come from a cool, outside source, rather than the hot engine compartment. Cold air is denser, and carries more oxygen than warm air, and it is oxygen that mixes with the fuel to create the power.
The headers are a less restrictive set of exhaust manifolds. The easier the air enters the engine, and the exhaust leaves the engine, the more power you have.
The H pipe is the part of the exhaust that the headers bolt to, and that contains your catalytic convertors. Various aftermarket versions are availble that flow better than the stock H pipe.
Cat back refers to the rest of the exhaust, from the catalytic convertors to the tailpipes, including the mufflers. These too come from aftermarket companies with less restrictive designs.
Hope that helps!
Take care,
~Chris