An override is an admin's ability to temporarily become an existing affiliate. In NATS3, overriding as an affiliate lets you specify settings for just that affiliate in NATS's administration interface. In NATS4, overriding as an affiliate doesn't affect the administration interface. In both versions of NATS, any changes made in the affiliate area change the affiliate's account.
In NATS3, if you don't see any action items in the Programs Admin, check your override status. As a regular affiliate, you can only use the enable and disable actions icons. Click Off in the upper-right corner to leave override mode and see all the action icons.