Long Tail vs Pareto

Long Tail
The phrase The Long Tail was first coined by Chris Anderson in an October 2004 Wired magazine article to describe the niche strategy of businesses, such as Amazon.com or Netflix, that sell a large number of unique items, each in relatively small quantities. Anderson elaborated the Long Tail concept in his book The Long Tail: Why the Future of Business Is Selling Less of More.
A frequency distribution with a long tail — the concept at the root of Anderson's coinage — has been studied by statisticians since at least 1946. The distribution and inventory costs of these businesses allow them to realize significant profit out of selling small volumes of hard-to-find items to many customers, instead of only selling large volumes of a reduced number of popular items. The group that purchases a large number of "non-hit" items is the demographic called the Long Tail.
Given a large enough availability of choice, a large population of customers, and negligible stocking and distribution costs, the selection and buying pattern of the population results in a power law distribution curve, or Pareto distribution. This suggests that a market with a high freedom of choice will create a certain degree of inequality by favoring the upper 20% of the items ("hits" or "head") against the other 80% ("non-hits" or "long tail").
The Long Tail concept has found a broad ground for application, research and experimentation. It is a common term in the online business and mass media, but also of importance in micro-finance (see Grameen Bank), user-driven innovation (Eric von Hippel), social network mechanisms (e.g., crowdsourcing, crowdcasting, Peer-to-peer), economic models, and marketing (viral marketing).

Pareto
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 events, roughly 80% of the effects come from 20% of the causes. Business management thinker Joseph M. Juran suggested the principle and named it after Italian economist Vilfredo Pareto, who observed that 80% of the land in Italy was owned by 20% of the population. It is a common rule of thumb in business; e.g., "80% of your sales come from 20% of your clients." Mathematically, where something is shared among a sufficiently large set of participants, there will always be a number k between 50 and 100 such that k% is taken by (100 − k)% of the participants. However, k may vary from 50 in the case of equal distribution to nearly 100 when a tiny number of participants account for almost all of the resource. There is nothing particularly special about the number 80, but many systems will have k somewhere around this region of intermediate imbalance in distribution.
The Pareto principle is only tangentially related to Pareto efficiency, which was also introduced by the same economist, Vilfredo Pareto. Pareto developed both concepts in the context of the distribution of income and wealth among the population.