Convoys are specific to either to North Atlantic or Arctic oceans, as well as to a certain year.
Some convoys are worth a lot of points, others give winning players additional reinforcements.
In each hand, a player can come out as German or Allied, so players will be constantly switching sides during the game. The person who contributed the most to the winning side gets the convoy and any sunken ships. The Arena, though both are equally fun. Atlantic Storm is definitely the more involved of the two games, but not by too much.