The Mark module allows users to "mark" any kind of content that is accessible to Views. Mark allows content to be "marked" per-user or globally. The module provides blocks to mark (and un-mark) nodes and users as well as a View field that can be used on any type of view. Mark is designed to be used by authenticated users. It's possible to assign the proper permissions for marking content to anonymous users, but the behavior of the module in such circumstances is "unspecified", at best.
Mark leverages the VotingAPI and Views to allow for many kinds of 'marked content' listings. Mark uses Chaos Tools to provide exportables and Features support.
How is mark different from Flag/Nodequeue?
Mark is, first and foremost, a VotingAPI module. VotingAPI does a the heavy lifting of recording votes, tallying them and providing Views integration.
Mark is Views-centric. Modules don't need to provide both Views integration and Mark integration. If a module provides new "base" tables Mark just works.
Mark doesn't have capacity for explicitly ordering things. Of course you can always sort by the number of people who have marked something, or by the time it was marked.
Mark, Flag and Nodequeue do have some conceptual overlap, and can be used to address similar problems. Mark is particularly good choice if you; need any of the features of VotingAPI, or need to mark content that isn't nodes or users.