List segmentation is used by marketers to send communications which are relevant to the set groups that it would be relevant to. Marketing would be specialized and tweaked for each segment of the list.
This helps to drive a high response in an email marketing message through sending messages that are relevant. Lists can be split in any way that the user would like to see such as long term and short term customer, amount of past purchases, area or geographical location, or price of product bought. Any way that can be defined can be used to segment such a list.
One example of when you might choose to split your email list is when a retail store plans a special local in-store sale that is not carried over to the online store. Here, you would want to segment your email marketing list by geographic location and send the marketing message to subscribers who live in the general area and avoid sending the message to your online-only shoppers would not be interested in the promotion. If a web site is representative of a number of shops but only one shop is running a promotional offer, the list of users of the site will be split so that only those users that use the shop running the promotion will be notified.