Here's a quick look at Batman: Arkham Knight's Red Hood story pack, which is a pre-order bonus at GAME in the UK.
The advert below is for GameStop, which has the same pre-order exclusive in the US, but the content is the same.
If you don't order from GAME then do not despair - completionists will be able to buy the pack separately at some point after the main game launches.
Additionally, GAME pre-orders come with the Harley Quinn Story Pack, but this is true of pre-orders elsewhere and isn't exclusive to the chain.
Alternatively you can just wait and buy the inevitable Game of the Year Edition of Batman: Arkham Knight, which will likely contain all of the above and more.