At least two quite different types of mailing lists can be defined: the first one is closer to the literal sense, where a "mailing list" of people is used as a recipient for newsletters, periodicals or advertising. Traditionally this was done through the postal system, but with the rise of e-mail, the electronic mailing list became popular.

When similar or identical material is sent out to all subscribers on a mailing-list, it is often referred to as a mailshot or blast. A list for such use can also be referred to as a distribution list.

In legitimate (non-spam) mailing lists, the individual can subscribe or unsubscribe themselves.

Mailing lists are often rented or sold. If rented, the renter agrees to use the mailing list for only contractually agreed-upon times. The mailing list owner typically enforces this by "salting" the mailing list with fake addresses and creates new salts for each time the list is rented. Unscrupulous renters may attempt to bypass salts by renting several lists and merging them to find the common, valid addresses. Mailing list brokers exist to help organizations rent their lists. For some organizations, such as specialized niche publications or charitable groups, their lists may be some of their most valuable assets, and mailing list brokers help them maximize the value of their lists.

Reference: Wikipedia
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