Gold

Gold is the primary resource in the game, you use gold for almost everything, such as training troops, upgrading troops, research, buying mercenaries among other things. This is the resource you will probably have the most of and the resource you will spend the most.

Elixir

Elixir is the blue bottle icon resource and it is used to produce some units and research upgrades in the academy. You will likely have an abundance of Elixir unless you play lich as some of their units cost Elixir to produce instead of gold.

Lumber

Lumber is primarily used to build buildings and upgrade buildings, but it is also used to research some upgrades in the academy.

Blood Diamond

Blood Diamonds is the red secondary resource which is used to build advanced units such as Silver and Gold tier units. Blood diamond is common compared to its sister resources of mithril and crystal. This is due to the fact that most of the units built by blood diamond are not staple units for any race and thus everyone usually has a ton of blood diamond laying around, which is great if you specialize in the units that do use blood diamond.

Mithril

Mithril is the green/turquoise colored bar icon resource. It is the rarest of the three advanced resources. This is mostly due to how few mithril mines there are early game and how far away the late game ones are from the capitals, thus making the mines a long cooldown. The other reason why Mithril is so rare is due to it being needed by every race to produce their frontline units. (the most likely unit to die and need replacing) You will find there will be a shortage mithril and players will attack you to plunder all and any mithril you have.

Crystal

Crystal is a fair balance of being needed for staple units while having a lot of mines to produce it. I don’t find there is a shortage of crystal like mithril but it is also not super common like blood diamond.