Pausing a Campaign stops all data collection (rankings, site crawls, link tracking, reports, etc.) but leaves existing data accessible. Paused Campaigns do not count towards account usage (keyword credits, crawl credits, etc.), and can be activated again at a later date. Since deleting a Campaign permanently deletes all data associated with the Campaign, pausing a Campaign instead of deleting it can better option in many cases -- especially if you think you may need the data again someday.