Black-Belt....how is the Black-Belt earned?

Black-Belt (with different Dans) is supposed to indicate highest or second highest level of mastery of martial arts in Judo, Taekwondo and Karate right? Just like to find out how skilled a Black-Belt Dan 1 is (how does a person gets there)? And how does one gets promoted to higher levels? Is it being misused such that one could reach Black-Belt pretty fast without being really at that level? What percentage of people among those that took up the sports actually are Black-Belts? Thanks in advance...