can be used to represent different effects on a card, especially those that last beyond the current turn. 
Examples include +1/+1 counters, charge counters, and time counters. Richard Garfield
created counters for Alpha
to designate an increase in power
, to track the gaining of resources, and to mark a permanent change. 
- See also: Counter (marker)/List of Counters.
From the Comprehensive Rules (Born of the Gods)
- 121. Counters
- 121.1. A counter is a marker placed on an object or player that modifies its characteristics and/or interacts with a rule, ability, or effect. Counters are not objects and have no characteristics. Notably, a counter is not a token, and a token is not a counter. Counters with the same name or description are interchangeable.
- 121.1a A +X/+Y counter on a creature or on a creature card in a zone other than the battlefield, where X and Y are numbers, adds X to that object's power and Y to that object's toughness. Similarly, -X/-Y counters subtract from power and toughness. See rule 613.3.
- 121.1b The number of loyalty counters on a planeswalker on the battlefield indicates how much loyalty it has. A planeswalker with 0 loyalty is put into its owner's graveyard as a state-based action. See rule 704.
- 121.1c If a player has ten or more poison counters, he or she loses the game as a state-based action. See rule 704. A player is "poisoned" if he or she has one or more poison counters. (See rule 810 for additional rules for Two-Headed Giant games.)
- 121.2. Counters on an object are not retained if that object moves from one zone to another. The counters are not "removed"; they simply cease to exist. See rule 400.7.
- 121.3. If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 and N -1/-1 counters are removed from it as a state-based action, where N is the smaller of the number of +1/+1 and -1/-1 counters on it. See rule 704.
- 121.4. If a permanent with an ability that says it can't have more than N counters of a certain kind on it has more than N counters of that kind on it, all but N of those counters are removed from it as a state-based action. See rule 704.
- 121.5. If an effect says to "move" a counter, it means to take that counter from the object it's currently on and put it onto a second object. If the first and second objects are the same object, nothing happens. If the first object has no counters, nothing happens; the second object doesn't get a counter put on it. If the second object (or any possible second objects) is no longer in the correct zone when the effect would move the counter, nothing happens; a counter isn't removed from the first object.
- 121.6. If a spell or ability refers to a counter being "placed" on a permanent, it means putting a counter on that permanent while it's on the battlefield and also to that permanent entering the battlefield with a counter on it as the result of a replacement effect (see rule 614.1c).