We have had a couple tickets regarding confusion on expiring content and the archive functionality and what is the expected behaviour.
When working in Episerver content can be set to expire by clicking Tools > Manage Expiration and Archiving in the content properties. On expiration the content will no longer be visible to users from the front end of the site. The content can either be set to expire immediately or for a later point in time.
The archive functionality can be used to relocate content to a new directory if it is desired for the content to still be visible. An example being a newsletter that should still be accessible to users, but needs to be labeled clearly as being old information. Within Tools > Manage Expiration and Archiving, a location for expiring content can have an archive location specified.
Note that when the content is archived that the content will not be relocated to the specified directory until the Archive Function scheduled job is run manually or setup to run automatically. Once the content is moved to a new directory through the archive functionality the expire date is removed from the content, and if it is desired for the content to be expired this can be manually set again.
More information on versioning can be found within the Working with versions section of the user guide.