What are Booster Packs?
You can find all your Booster Packs under the 'Booster Packs' tab in your storage room. They contain a random assortment of items ranging from resources to units.
These packs aren't completely random, though. Every potential Booster Pack item is rated from common to rare: common includes resources and units at the same level as the player, while rare includes licensed units you could normally only get from the Black Market, or sometimes not even from there!
How to get and use Booster Packs
Currently players receive one Booster Pack for every two levels they gain. These 'Mysterious Packs' contain two common items, two uncommon items, and one rare item - rare meaning a licensed unit. There are also Crates which can be acquired from Bosses, and a few other special packs (such as the Advent Packs that were given away at Christmas).
Using the Booster Packs is easy, as you'd expect. You will usually need to pay a few Gems to open the Packs. When you press the 'Open' button, the Gems required to open the Booster Pack will immediately be removed from your account and you will get your items at once. The used pack will then vanish from your storage.