Pareto principle

The Pareto principle (also known as the 80-20 rule, the law of the vital few and the principle of factor sparsity) states that for many phenomena, 80% of the consequences stem from 20% of the causes. It was named after the Italian economist Vilfredo Pareto, who observed that 80% of income in Italy was received by 20% of the Italian population.

The idea has rule-of-thumb application in many places, such as:

Work - 20% of your efforts produce 80% of the results.
People - 20% of the people you know (friends, colleagues, family) provide you with 80%
of nurturing support and satisfaction.
Business - 80% of the sales are made by 20% of the sales team.
Focus - 20% of your activities will account for 80% of your success.