Creature

In Magic: The Gathering, creature is a permanent card type.

Flavorwise, creatures represent Warriors, Minions, Beasts, and Monsters that serve the player, usually by fighting on their behalf. Because almost all creatures can attack each turn to reduce an opponent's life or block the opponent's attackers, creature cards are fundamental to most deck strategies.

Description
Creatures are played on the player's own main phase when the stack is empty. When a creature comes into play or changes controllers, it has what is commonly called "summoning sickness" until the beginning of its controller's next turn. A creature with summoning sickness cannot attack or use an activated ability with the tap symbol in its cost, but it can block or use any other abilities it has. A tapped creature cannot attack, block, or become tapped for and/or fulfilling a cost.

On the bottom-right corner of each creature card is that creature's power and toughness, respectively. The power is the amount of damage a creature deals with an opponent or other creatures in combat, and the toughness is the amount of damage a creature can survive. A creature with damage equal to or greater than its toughness has "lethal damage," and is destroyed. Similarly, a creature whose toughness is reduced to zero or less will go to its owner's graveyard (though it is technically not destroyed.) Any damage a creature takes will accumulate until the end of the turn when all damage is removed from all creatures.

Unlike other card types, almost all creature cards have a subtype, also referred to as a "creature type." There are no rules inherent to creature types, but there are many cards that affect specific types. In addition, creature types are often associated with particular colors and abilities, typically for flavor purposes. For example, Angels are almost always large white flying creatures, Spiders are typically green creatures with high toughness and reach, and Goblins are often small red creatures with self-destructive abilities.

Faster, stronger, better
Creatures are nowadays better than they were ten or fifteen or twenty years ago. In Alpha there was a fundamental imbalance in power between creatures and spells. Since Alpha there has been an ongoing effort to make creatures stronger in comparison to non-permanents.

Small, medium, large
The Mystery Booster test card Scaled Destruction defines small creatures as having a total power and toughness 4 or less, medium creatures have 5-8, and large having 9 or more.

Modified
The Mystery Booster test card Louvaq, the Aberrant defines modified creatures as having a power, toughness, or ability different than their printed versions.

Subtypes
The subtype for creatures is called creature type and shared with tribals.

Friendly to creatures
Green and white are the main creature colors (they have the highest percentage of creatures versus spells) so they most often like you having creatures (they reward you for playing them, having a certain number on the battlefield or in your hand, etc.).