Gold : a premium, multi-purpose currency which is used to buy things in the shop.
Silver : a currency which mainly used to build/upgrade buildings
Iron : a currency which mainly used to upgrade troop technology
Experience : to upgrade your generals’ levels
Rations : your troops need Rations each time they go to battle
There is no limit on how much you can hold any resources, except Rations. Rations is capped at 240 (can be further increased to 960 by buying Quality of Life feature in the Shop).