A feature is a collection of Drupal entities which taken together satisfy a certain use-case.
Features provides a UI and API for taking different site building components from modules with exportables and bundling them together in a single feature module.
A feature module is like any other Drupal module except that it declares its components (e.g. views, contexts, CCK fields, etc.) in its .info file so that it can be checked, updated, or reverted programmatically.