Paranoia is a re-implementation of [acts\_as\_paranoid](http://github.com/technoweenie/acts_as_paranoid) for Rails 3, using much, much, much less code.
You would use either plugin / gem if you wished that when you called `destroy` on an Active Record object that it didn't actually destroy it, but just "hid" the record. Paranoia does this by setting a `deleted_at` field to the current time when you `destroy` a record, and hides it by scoping all queries on your model to only include records which do not have a `deleted_at` field.