There are 3 different terms:
"Commander" is any unit currently leading a detachment. It is them you have to be 12" away from to be in command. The commander is automatically the highest ld model or you may choose any character.
"Warlord" is a commander who is a character. The only difference is you get a card for each warlord alive at the beginning of a turn.
"Warmaster" is your overall army commander. They get a random warlord trait and are important in some missions