Bullets
The pemium currency in The Walking Dead: March to War is bullets. You can either earn them by completing Council stories or purchase them using real money by tapping the icon. You can spend bullets in the market to obtain resources, powerups, speedups, and defensive bonuses. They basically speed up certain processes in the game to help you progress faster than you normally would.
Core Currencies
The core currencies, particularly Food and Salvage, are what you use to build yourself up and increase your strength. They are introduced at the start of the game so you will learn early on the significance they have to the systems of the game. Core currencies are used for any constructive action such as constructing and upgrading the various buildings in the base and anything involving recruiting and maintaining Survivors.