From Josh Morony: "It is quite common in mobile games to provide a shop where players can purchase items and upgrades for the game. It might sometimes be called a store, upgrade center, barracks or something else but the general idea is the same: a screen where users can purchase things that affect their experience in the game some way.
A lot of the time this is integrated with In App Purchases, which allows you to charge real money for items that are purchased in the shop. It is also quite common to offer items in shops that use in game currency that the player can earn without spending real money too.
A combination of both of these is usually a good approach. You may allow the user to redeem experience points earned in battles for items and upgrades, but certain items will require using real money for example.
This allows people to enjoy the game for free – people who can not afford to pay or who do not think it is worth paying for will help drive the growth of a game. People who are willing to spend real money then help the game become profitable."