There are four different kinds of resources in Terraforming Mars: Ares Expedition: mega credits, heat, plants, and special resources.
MEGA CREDITS (MC) - MC is used to play cards and activate actions.
HEAT - Heat is used to raise the temperature of Mars. During the action phase, players may spend 8 heat to raise the temperature one step and gain 1 TR.
PLANTS - Plants are used to make forests, which raise the oxygen on Mars. During the action phase, players may spend 8 plants to gain a forest VP, raise the oxygen one step, and gain 1 TR.
SPECIAL RESOURCES - Some cards allow you to place special resources (animals, microbes, and science) on that or other cards. These resources do different things, as specified by the different cards.