The rule was discovered by Vilfredo Pareto, an Italian economist who noticed that the division of wealth in Italy at the time fell into a clean 20% owning 80% of the wealth. He then thought to look at where else this breakdown might apply and started finding it as a pattern for many things.
Later, modern project management theorists noticed it in their efforts to get work done.
And today, the principle is applied even to the job search and life management in general.
Here's an article I read that does a pretty good job of explaining it. Enjoy!
How the 80/20 Rule Can Help Improve Your Productivity!