Here's the description from 3.5.
Marshals inspire trust in those they lead. They earn that trust by slogging through harsh landscapes, dangerous battlefields, and haunted catacombs along with those under their command. With a look, they can see where to best deploy their resources or come up with a sneaky ruse to fool their enemies. A marshal has a tactician's mind, a cartographer's overview of the disputed landscape (or dungeon warren), and a way with words that can inspire battle-hardened fighters to give it their all when melee breaks out.
Trained in the basics of fighting, marshals possess a general knowledge of weapons and armor. Their real strength is their ability to lead those who follow them to success they might not otherwise reach in combat. Marshals make passable warriors themselves, when personal danger finds them.
And from 4e.
Warlords are accomplished and competent battle leaders. Warlords stand on the front line issuing commands and bolstering their allies while leading the battle with weapon in hand. Warlords know how to rally a team to win a fight. Your ability to lead others to victory is a direct result of your history. You could be a minor warchief looking to make a name for yourself, a pious knight-commander on leave from your militant order, a youthful noble eager to apply years of training to life outside the castle walls, a calculating mercenary captain, or a courageous marshal of the borderlands who fights to protect the frontier. Regardless of your background, you are a skillful warrior with an uncanny gift for leadership.
Your leadership takes the form of quick commands, cunning strategies, and tactical superiority. Your powers guide your allies to extra and more powerful attacks, as well as helping them move quickly in combat situations. You also assist your allies by moving your enemies around or knocking them prone. You use Strength for your attack powers, so make that your best ability score. Intelligence is secondary, because your Intelligence determines just how effective a leader you are. Charisma should be your third best score