as more air/fuel ratio goes into the engine (when you press the gas and the throttle blade opens) It makes the engine have more fuel mix which in turns makes the pistons move up and down faster and faster. Check out places like howthingswork.com (i think that is the right address). Oh and welcome to honda swap. Stick around and you will learn a shit load
The ECU (car's computer) is what controls the rev limiter. If you rechip your ecu you can (but its not recommended) get it to rev higher. If you rev much past the red line on a stock engine, it is likely that you will get "valve float", and this can cause a valve to smash into a piston. Also, without different cams, you will lose power really really fast when you go past the red line.
so basically if you want to rev higher, you will need:
A rechipped ECU
New valve springs, retainers, and possibly new valves.
just a note...the reason why a f20c head (s2000) does not get valve float past 7200 rpms, is because the valve springs are a lot stiffer than the civic's, so they are able to open/close faster. Just another note, with stiffer valve springs, you are making the valve hit harder each time it closes, so stiffer valve springs can also cause valves to wear prematurely...thats why its a good idea to upgrade those too.