Command & Conquer is a RTS game with two campaigns. Your objective in them is to defeat the enemy with any available resources.
Between missions, some credits you have stockpiled may carry over to the next one. However, some missions do not use credits, and as such any stockpiled resources are lost.
Overall, GDI units are stronger than Nod's, but are more expensive, and also slower whereas Nod units are fast, cheap and stealthy. This allows the Nod player to outmaneuver the GDI player whereas GDI can just outgun Nod. Nod is also more reliant on infantry (because of the need to support their weak vehicles) where GDI is more reliant on vehicles (mainly tanks).