CVG previews the roles and the multiplayer modes in World in Conflict:
Each unit has its secret nemesis, its militaristic phobia, its dark strategic allergy. A building filled with dug-in infantry falls prey to long-range artillery controlled by whoever chooses the support role (each team is commanded by anything up to eight players, each choosing one of four roles). That long-range artillery is susceptible to tank attacks (controlled by the armour role), and similarly, those tanks will be wasted by a quick-thinking heavy helicopter (air role, and the final one is the infantry role, if you haven't already realised).
The role you choose determines which units you can purchase (although it's possible to buy units from other roles at a premium), and your purchases are airdropped in at regular intervals. You're only ever commanding a small clutch of units too, and that means your attention is never spread too thinly.