There are three main types of Resources in the game: Fuel, Munitions and Rations. You need these Resources to construct Buildings, research Tech and to hire and maintain your Units.
Mercenaries in game Soldiers Inc: Mobile Warfare are special Units you can collect and improve. Each has a set of Skills that give different boosts to your Base and/or armies. This is different from Strategic Units, which improve the statistics of only a certain number of Units.