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.
Litemind at http://litemind.com/ is an excellent website setup and run by Luciano Passuello in Brazil. Check it out! Perhaps the most valuable item on his website is the free e-book (upon registering) entitled "The Very Best of Litemind." This book is full of great ideas. I have drawn a Mind Map of it.