RFC 2369 - List Unsubscribe Headers
The mailing list command specification header fields are a set of structured fields to be added to email messages sent by email distribution lists. Each field typically contains a URL (usually "mailto") locating the relevant information or performing the command directly.
By including these header fields, list servers can make it possible for mail clients to provide automated tools for users to perform list functions. This could take the form of a menu item, push button, or other user interface element. The intent is to simplify the user experience, providing a common interface to the often cryptic and varied mailing list manager commands.
The List-Unsubscribe field describes the command (preferably using email) to directly unsubscribe the user (removing them from the list).
Helpful Information: http://www.ietf.org/rfc/rfc2369.txt