Details on Commando's game modes have been just released:

Single Player Campaign
In singe player mode, you control a GDI commando called Jack "Havoc" Sheppard.
Your Commander will assign you different kinds of missions including demolutions and eliminating enemy soldiers.

Skirmish
In Skirmish, the objective is to destroy all enemy units and structures. Commanders can build structures and assign other player their own missions.
The Commander who manages to destroy the enemy is victorius.

Capture the Flag
In Capture the Flag, each Commander has a flag that is attached to Mobile Construction Yard vehicle.
When Commander starts building the base, the flag drops down in front of the construction yard.
The flag is picked up by driving a vehicle inside the enemy's pickup zone
and when the flag is delivered to friendly pickup zone, the enemy will lose.
Note that picking up the flag will slow down the carrier vehicle.
Check out the rest of the modes here.