It was developed right after World War II by Willard F.
Shy of a date stamp on an object, it is still the best and most accurate of dating techniques devised.
Given relatively pristine circumstances, a radiocarbon lab can measure the amount of radiocarbon accurately in a dead organism for as long as 50,000 years ago; after that, there's not enough C14 left to measure. Carbon in the atmosphere fluctuates with the strength of earth's magnetic field and solar activity.
You have to know what the atmospheric carbon level (the radiocarbon 'reservoir') was like at the time of an organism's death, in order to be able to calculate how much time has passed since the organism died.
Trees maintain carbon 14 equilibrium in their growth rings—and trees produce a ring for every year they are alive.
Although we don't have any 50,000-year-old trees, we do have overlapping tree ring sets back to 12,594 years.
The half-life of an isotope like C14 is the time it takes for half of it to decay away: in C14, every 5,730 years, half of it is gone.