Groups of placements that share schedule and pricing information and are associated with the same site.
A package can be used for any group of placements. Reports show the sum total of impressions served to all of the placements in the package.
A roadblock is typically a group of placements on one webpage. In most cases, a roadblock arrangement is meant to ensure that only one campaign is displayed at any given time on that webpage. The grouping of placements serves as a "roadblock" to prevent ads from other campaigns from being served.