A marker is used to represent anything which isn't represented by a card.
The most common marker are tokens, emblems, and counters. Tokens and emblems are usually represented by the marketing cards found in magic booster, while counter are often represented by marbles or dice.
Some designations are described "to act as marker":
Some designations are not described as a marker by the rules:
The Grand Melee variant places great emphasis on turn markers.