Leader

Background
Hero Units are UNSC and Covenant Combat Personnel in the upcoming game Halo Wars who are suited to lead Marine and Covenant ground forces along with air units such as Dropships, Hornets, Banshees, etc.

Known Hero Units
UNSC

Captain Cutter

Sergeant John Forge

SPARTAN-II Alice

Covenant

Prophet of Regret

Unknown Arbiter

Unknown Brute Chieftain.

Combat Experience
Being that they are 'hero units', they are more adept to fighting and have more armor for protection against the enemy fighting units. Along with that, they probably have more unique weaponry and have "special abilities" such as "Rage mode" for the Arbiter.

Triva

 * In one of the lastest videos, one of Ensemble revealed ther are only 6 Hero units, 3 for UNSC and 3 for the Covenant.
 * If the above statement is true, it could be that the character cards in the Halo Wars LE represent the six hero units, meaning Spartan Alice is not a hero unit. On the contrary Anders would be the last one.