The module classifies inactive users into two categories: users who have never logged in before, and users who have logged in at least once.
For users that have never logged in before, the user can choose to prune users based on how long they've been registered.
For users that have logged in before, he can chose to prune users based on both how long they've been registered, and how long its been since they last logged in.
Pruning specification can be selected and be saved as a cron job, or executed a single time.
Unpack in your modules folder (usually '/sites/all/modules/')
Enable under Administer > Site Building > Modules