The Cash Shop is where players can buy items with real money to gain an edge over other non paying players.
Items such as digi-eggs, data, evolution items and cosmetic items can be found here.
Digi-eggs and data can be found in-game although it is a much longer process than buying them from the shop.
Evolution items such as those used to unlock some of the burst evolution forms can only be obtained via the store.
The cosmetic Items are purely for customising your character.