What's the difference between a 'Series' and a 'Collection'?
A Collection are audiobooks grouped together around a common theme, like a reoccuring character, creature type or plot device. Because of this, they can usually be listened to in any order.
A Series is a sequence of stories which lead on from eachother and should be listened to in the order they are listed. While individual stories within a series will usually make some sense without listening to prior entries, it is reccomended that you listen to them in the order they are listed.