The Pareto or 80-20 rule is a rule of thumb that things such as "about 80% of sales comes from 20% of your customers", "about 80% of complaints come from 20% of the people", "about 8o% of sales come from 20% of the products", and "80% of your effort should be directed to your top 20% of priorities."
The Pareto rule is related to a statistical distribution of the same name.
According to Wikipedia, quality-management guru Joseph Juran (who I heard speak in Toronto years ago) suggested that this rule be named after Italian economist Vifredo Pareto (1848-1923).
One good example of the Pareto rule is the distribution of economic activity, measured by Gross Domestic Product (GDP) in the world. The top 23 countries account for 80% of total world GDP (see file/ graph and map).
The take home message of the 80-20 rule is that one should focus on those customers, products, or activities that account for the majority of profits, and not to get distracted by the minor ones.