Commander I. Heartly Noah here.
If there is one thing that I Noah, it's Commanders.
Commanders are units that can lead troops in battle. You need one to give orders in battle; otherwise units will automatically attack when in contact with non-allied units. Note: not enemy units, non-allied. And they don't only fight blindly, they HAVE to fight. If you want to sit and chat or have tea with some non-allied units, both sides had better have a Commander present. If you have any tactics or strategies to implement in battle, you need a Commander on the ground.
Warlords are Commanders that have the Leadership natural ability. This is expressed as a bonus modifier that is imbued on all units that are stacked with them. All stacked units get a bonus up to 8 per stack, but a Warlord's Leadership applies to every unit in a stack, even if that stack is hundreds large. Warlords are who you want leading your units into battle, and almost always are the ones to do so, which is why some people think that Warlord and Commander are the same thing. But that's wrong. Every Warlord is a Commander, but not every Commander is a Warlord. Because...
Casters are Commanders too. A Caster doesn't have a Leadership bonus, but instead gets to cast spells. Casters are also generally weak and are rare and valuable, and can be more easily turned after capture than a Warlord. For these reasons, they almost never lead troops in battle. But, as noted above, if you don't have any Warlords, a Caster can still guide your battle. And some Casters have special bonuses leading certain troop types. So in rare circumstances you might have a Caster lead a stack, but you wouldn't want to be in any of those circumstances.
Finally, for the sake of completeness, the Chief Warlord. Any Warlord can be named (and presumably unnamed) Chief Warlord. It is unknown if you can make a Caster a Chief Warlord; it wouldn't help anyway. The Chief Warlord provides a Leadership bonus to every unit in the hex, and an additional bonus to the units in his stack. It is unclear whether the hex bonus or the stack bonus is his normal Leadership modifier.
I am a: Chaotic Neutral Human Bard/Sorcerer (2nd/1st Level)
Str- 12, Dex- 15, Con- 12, Int- 14, Wis- 11, Cha- 13