This is kinda basic, sorry

Getting new cams changes how long they keep the valves open. The lobes on the cams hit the rockers which open the valves, and you can grind cams differently to keep the valves open for longer periods of time and at different times. An aggressive set of cams will usually have a good amount of overlap, meaning the intake and exhaust valves are open at the same time.

Cam gears i'm not quite as sure about, all i can basically say is they change the timing of the cams so that you can make them more agressive at low rpms or at high rpms. All they do is move power around, they really give you gains.

Hope that helps some, sorry if it's really basic.