There are three Factions available to play as in Heroes and Generals, each one has its own specific weapons, equipment and vehicles. These are 
Germany, 
United States and 
Soviet Union.
You can play any three factions as much as you want in staged battles, however if you want to take part in the war you will need to make a choice between the three as once you join one you cannot switch until the war is over.
Each team has their own set of unique weaponry, vehicles and other equipment which is different from the other team's arsenal, so choose carefully before you take a side.