JEDEC -
http://en.wikipedia.org/wiki/JEDEC
It's an industry standard that everyone follows to make stuff work with each other.
The simplest way i can think of to explain RAM frequency is like this.
Imagine you have 3 cars.
A toyota corolla (cheap), a ford mustang(little expensive) and a ferrari (very expensive)
When you drive them on the highway you have to follow the speed limit 110 km/h
doesn't matter what car you get, all of them MUST obey the speed limit.
this is how ram works.
you can buy 1333MHz ram(cheap) or 1600MHz ram (little expensive) or 2666MHz ram(very expensive)
by default (no overclocking) all of them run at 1333MHz - this is the default DDR speed (your normal speed limit)
if you want a ferrari to go at 250 km/h you have to step on the gas.
if you want your 2666MHz ram to operate at 2666MHz, you have to overclock the system.
by default your 2666MHz ram will operate at 1333MHz when plugged into a system and it will have no performance benefits over regular ram operating at 1333MHz
its like having a ferrari driving at 110km/h on the highway, yes you will look good getting there but the corolla will get you there in exactly the same time. The difference only comes into play if you actually speed or in the RAM's case, you overclock.