What is the difference between Passive, Active and Student headsets?
Passive, or "Passive Noise Reduction" (PNR): loads of foam to dampen the sound. Simple really.
Active, or "Active Noise Reduction" (ANR): Electronic wizardry that listens to outside noise and then generates the exact same noise but in opposite phase, effectively canceling the outside noise. It's something you've got to hear before you believe it. It works very well but the main disadvantage (apart from the price tag) is that an active unit always needs a power supply. The standard two-lead connector doesn't supply this so you either need to have a battery unit integrated in the headset lead, with finite life before the battery runs out, or a special (round five-pin?) connector in the panel, wired into the aircrafts power system.
Some headsets such as the Bose-X are active only. Meaning that if they lose the power supply their passive insulation is very low. Others, such as some Sennheisers and DC Clarks, combine active and passive: Loads of foam and ANR. Should the ANR quit, either due to a technical malfunction or a loss of power, you are left with a pretty good passive headset.
Because of this the clamping force on passive or passive+active headsets is usually more than active-only: On active-only getting a proper seal around your ears is less important.
Student headset - simply a low-budget passive headset.
I personally own a (passive) SkyLite SL-900 bought over the internet (eBay) from an outfit in Hongkong. Including taxes, shipping and everything I spent about 130 euros. Very happy with that one. My first headset was an Avcomm AC-200PNR which I bought at a US pilot shop while I did my training there, for a little over 100 dollars I think. Also a very reasonable unit. It's my backup unit and used for passengers. The difference between the two is marginal and comes down to comfort, really. Very much a personal thing.