A charge counter is just a certain type of counter. All counters used in magic have names to distinguish them from other types of counters. A card could have many different types of counters on it at once time, so cards need to reference the name of the counter...
Say a card had 2 charge counters and 1 +1/+1 counter on it. Though the card has 3 counters total, only the charge counter would only count for something like "You win the game when this card has 20 charge counters on it"
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