When you archive an article, the content is put into a state which removes it from your site as published content. The article is still available from within the administration interface and can be retrieved for archival purposes. Trashed articles are just one step from being deleted but are still available until you remove the items from the trash manager. You should use archive if you consider an article important, but not current. Trashed should be used when you want to delete the content entirely from your site and from future search results.