Cam Shafts simplest description is, that they open the valves. The importance of that cant be stressed enough because its not just about opening valves its when but how long for & how much lift (length-depth) the valves get opened for. Manufactures design cam shafts for a number of purposes ie a road bike, dirt race etc etc & in just those few phrases there are many sub categories but when it comes to Tuning you can alter the timing between the 4 strokes & valve moments by dialling the cams in! Often its barely possible to see the difference by the human eye so dial (drop) gauges & degree wheels are used. But unless you have significant experience with the type engine your working on to get the magic numbers you will need access to 10's of thousands of pounds worth of equipment including dyno facilities air flow meters real world ram air.