Are they worth it? What exactly do they do?

sry for the noob question.

 

Well in a d series there is only 1 Cam Gear, Search for Adjustable cam Gear

 

Yes they work, but dont recomend doing a cam gear unless your running big nitrous or turbo set up, or serious all motor.
The cam gear will allow you to open the valve sooner or later ie.,advance/retard., by degree. I also recomend running ignition set up with cam gears to be able to also adjust spark advance/retard.
If your just a basic street car with minor bolt on stuff, I dont think just a cam gear is much help alone IMHO.