I've received terrible feedback on this topic before, but damnit it's important.
Lets say you have a skill with 1 sec CD at 0% CDR. In 10 seconds that's 10 casts. Now lets add 40% CDR. You might think that allows 4 more casts, for 14 total. But it doesn't. The CD is lowered by 0.4 sec to 0.6 sec, which allows for 10/0.6=16.67 casts, an increase of two thirds.
As it works this way, it also means that each point of CDR is worth more than the previous, meaning the CDR you get from 0% to 10% is worth fewer extra casts(11.11%, one ninth) than the 10% from 30% to 40% (16.67%, one sixth), so the last 10% CDR is actually worth 50%(!) more than the first 10%.
As such, it is one of the three stats in League that scale synergistically with themselves, one other being percentage movement speed increase (which doesn't really count because there are two soft caps), and technically health regeneration, but the effect of that is so immensely miniscule.
It is also an often underrated multiplier for casters. I'll get into multipliers in a later blog.