Commanders are a feature that allows you to bring yourself into the battlefield. When commanders are enabled, all players in the game are given a special commander unit on one of their territories.
The commander represents you. You're on the battlefield with the rest of your armies, commanding them. If the commander dies, all of your armies change to neutral and you're eliminated from the game.
Commanders can attack, defend, and transfer in the same way that normal armies can. When you issue an attack or transfer order from a territory with a commander, a check-box will appear that allows you to optionally include the commander in that attack or transfer.
When in combat, commanders act like a stack of 7 armies. For example, if you attack with your commander along with 3 armies, it will do the same amount of damage that attacking with 10 armies would do. Similarly, if you are defending against an attack with your commander and 3 armies, you'll do damage to the attacker equal to if you were defending with 10 armies.
When a commander engages in combat along with armies, the armies always take damage first (they're protecting the commander with their life). For example, assume a territory has a commander plus 3 armies, and 10 armies attacks it. Using the default 60% kill rate, the 10 armies will do 6 damage. Since armies die first, the 3 armies die, and 3 damage would be left to be applied to the commander. Since it takes 7 damage to kill a commander, and commanders are never partially damaged, the 3 remaining damage does nothing. In order to kill a commander, all 7 damage must be done to it at once in a single attack.
In manual distribution games, commanders will always start on the first territory you are awarded (usually your #1 pick). In auto distribution games, commanders will start on a random territory of yours.
For the most part, commanders follow the same rules as armies. For example, in multi-attack games, commanders can attack many times, but as soon as they transfer they can't move again. However, unlike normal armies, commanders can not be transferred to a teammate (they must always remain on a territory you control.) Using an Airlift Card, commanders can be airlifted, as long as the destination is one of your own territories and not a teammate.
Only members may enable commanders for their games, however anybody can join a game that uses commanders.