Combat - the core of the game of Magic.  Most games of Magic are won and lost in the combat step, and almost every game of Magic you play will involve combat.  Most creature keywords and abilities affect combat as well, and we'll go over the most common abilities here as well.

The Combat Phase

If you remember, the combat phase happens after the first main phase of the turn, giving the player whose turn it is a chance to cast creatures, enchantments, artifacts, sorceries, and planeswalkers before they attack.  Combat is broken into the following steps:

  1. Beginning of Combat - At the beginning of the combat phase, both players get the opportunity to cast instants and play abilities.
  2. Declare Attackers - The player whose turn it is chooses which of their creatures (if any) will attack and what player or planeswalker they will attack.  Only creatures that the attacking player has controlled since the beginning of the turn and are untapped may attack.  Once attacks are decided, the attacking creatures tap as the cost for attacking.  Once all costs are paid, each player gets another opportunity to cast instants and play abilities.
  3. Declare Blockers - Any player that is being attacked or controls a planeswalker that is being attacked may choose to block the attacking creatures with their own creatures.  A creature can only block if it's untapped, and a creature can only block a single attacker.  Any number of blockers can be assigned to a single creature, so if Player A attacks with two creatures and Player B has three creatures, Player B can block one of the attacking creatures with two of their creatures and the other with one, or all three on one attacking creature letting the other go through unblocked, or they can block with some or none of his creatures instead.  Once the defending player has decided their blocks, any attacking creature blocked by two or more creatures has those blockers ordered.  The order determines how the attacking creature will deal damage.  After all of that, there is another opportunity for players to cast instants and play abilities.
  4. Damage - Unblocked attacking creatures deal damage equal to their power to the player or planeswalker they are attacking.  Creatures that are blocked by one creature deal damage equal to their power to the blocking creature and the blocking creature deals damage equal to its power to them.  Creatures that are blocked by multiple creatures deal damage to the first blocker up to their power but no less than what would be enough to kill the first blocker.  Any damage left over can be assigned to the next blocker in the same manner, and so on.  Let's look at a couple examples:
    • An attacking Enormous Baloth is blocked by two Siege Mastodons. The Baloth's controller can assign five damage to the first Mastodon and put the other two on the second Mastodon, put six damage on the first and one on the second, or put all seven damage on one. They cannot put four on one and three on the other.
    • Same situation, but this time, before damage, you play a Lightning Bolt on the first Mastodon. Now, since it already has three damage, you only need to assign two more damage to it to kill it, so you can assign the other five to the second Mastodon.
    • Same as the first situation, but this time, your opponent plays Giant Growth on the first Mastodon. It now has eight toughness, so you have to assign all seven damage to it. Once blockers are ordered in the declare blockers step, you can't change that order if something like Giant Growth happens to make the combat bad for you.
  5. End of Combat - At the end of combat, players get another opportunity to cast instants and play abilities.  Once everyone passes, the game moves to the next main phase.

Let's define a couple terms within the game, because you'll see them a lot on cards.

Attacking - A creature is attacking if it's declared as an attacker.  It stays attacking until all players have passed at the end of combat or it leaves the battlefield.
Blocking - A creature is blocking if it's declared as a blocker.  It stays blocking until all players have passed at the end of combat or it leaves the battlefield.
Unblocked - An attacking creature is unblocked if no creature was assigned to block it during the declare blockers step.
Blocked - An attacking creature is blocked if one or more creatures are assigned to block it during the declare blockers step.  If all creatures blocking it leave the battlefield, the attacking creature is still blocked.

"That doesn't make sense, James.  If nothing's blocking a creature, shouldn't it be unblocked?"  Think of declaring blockers as intercepting the oncoming attack.  Even if the blockers leave, that's given you the time to move to a new location and avoid the attack.

Combat would be pretty boring and lacking in strategy if it was just big creatures with no abilities running into each other.  Fortunately, there are a lot of abilities that affect combat and make the game far more interesting.

First Strike and Double Strike

Viashino Spearhunter from M10Creatures with first strike and double strike split the damage step of combat into two steps.  The first damage step is just for creatures with first strike and double strike - everything else has to sit there and wait (or get killed by the guys with first strike and double strike).  Then, in the second damage step, any creatures that are still around that haven't dealt damage (so you can't get cutesy and give a creature first strike after the first strike creatures have dealt damage to 'turn it off') AND creatures with double strike deal damage.  That means creatures with double strike deal damage twice in combat and can quickly decimate an army of blockers... or an opponent.  First strike and double strike are most often found in red and white.  Black also gets a little first strike, usually on dark knights.

Flying

Air Elemental from M10Creatures with flying can't be blocked except by other creatures with flying and creatures with reach.  This makes a creature with flying hard to block and a lot more valuable as an attacker.  This, along with similar abilities, is sometimes called an evasive ability, since it can evade most blockers and deal damage directly to players or planeswalkers.  Similar abilities are landwalk and trample.  Flying is most often found in blue and white, with black getting some as well.  Red gets a few splashy fliers as well (usually dragons), with green rarely getting this ability.

Trample

Creatures with trample deal damage differently than normal creatures.  If a creature with trample is blocked, it treats the player or planeswalker it's attacking as the last blocker in order.  So if a creature with five power and trample is blocked by a creature with two toughness, that creature can assign two damage to the blocker and trample the last three damage over to the player or planeswalker it's attacking.  Green creatures have trample as their most common form of evasion, as it allows their big creatures to get damage through small blockers.  Black and red creatures sometimes have trample as well.

Landwalk

Bog Tatters from ZendikarCreatures with landwalk can't be blocked if the defending player controls a land of the specified type.  So if the defender controls an Island, they can't block a creature with islandwalk.  This is usually a basic landtype (forestwalk, swampwalk, mountainwalk or islandwalk), but any land type can appear.  Dryad Sophisticate has non-basic landwalk, for example, and can't be blocked if the defending player controls non-basic lands.  You'll note I skipped over plainswalk, and that's because there are rarely any creatures with plainswalk - the flavor of the ability is that these creatures excel in blending into the land type they have landwalk for (dryads in the forest, tunneling creatures in mountains) and it's hard to blend into the open plains.  Most often, creatures with landwalk are found in the color those lands produce (forestwalkers in green, islandwalkers in blue, etc), but green can also get landwalkers of other types.

Reach

Giant Spider from M10Creatures with reach don't get any bonus when they're attacking, but are very effective blockers since they can block creatures with flying.  Most often templated as spiders or archers, these creatures, while stuck on the ground for attack, can envelop or shoot down flying creatures.  Green has the most creatures with reach, which makes sense, since it has the least fliers.  White also has the occasional creature with reach.

Deathtouch

Damage dealt by a creature with deathtouch, combat or otherwise, destroys that creature.  Because of this, when a creature with deathtouch is blocked by multiple creatures, it only needs to deal one damage to each to do enough to kill it, so a 2/2 with deathtouch like Acidic Slime can kill the first two creatures blocking it, no matter their toughness.  Deathtouch is almost exclusively found in green and black, representing natural toxins and deadly poisons wielded by creatures of these colors.

VIGILANCE

Jedit's Dragoons from Time SpiralCreatures with vigilance don't have to tap as a cost of attacking.  They still have to be untapped to attack, but they stay untapped and as such, they can block as well as attack.  That makes deciding whether or not to attack with a creature with vigilance a lot easier, as you'll still have him available to block on your opponent's turn unless it's killed in combat, and then it'll probably have taken a creature with it.  Vigilance is most often found on white creatures, as white is the best at defense through creatures and combat skill.  Blue and green also have some vigilance.

Haste

Creatures with haste get to ignore the rule that a creature has to be under a player's control since the beginning of the turn to attack.  They also get to play abilities with the tap symbol in the cost on the turn they come into play.  This gives creatures with haste an element of surprise, as normally your opponent has a turn to react to the creatures you play before you can attack with them.  Red gets the most creatures with haste, playing into their impulsive, straight ahead nature.  Green and black also get get haste creatures infrequently as well.

Lifelink

Caravan Hurda from ZendikarDamage dealt by a creature with lifelink causes its controller to gain that much life.  While not having a direct impact on combat, a creature with lifelink can make it harder for its owner to be killed by not only keeping an attacker from getting to the player, but gaining life with the damage it deals.  White and black have lifelink most often, white as a color of healing and protection, and black as a form of vampirism.

Defender

Creatures with defender often have better base stats than a comparable creature without defender, but that comes at a cost - creatures with defender can't attack.  As the name implies, they can only block.  Often, defenders will have other abilities that make the cost of not being able to attack with them one a deckbuilder is willing to pay.  You'll see defenders in all colors, with the most being in the defensive colors, white and blue, and the least in the aggressive colors of black and red.

If it feels like there's a lot going on, there is, especially with the more complicated abilities like trample, deathtouch, and first and double strike.  It's that variety of combat that draws players to the game of Magic, however, as it's a unique puzzle every game of how to best utilize your creatures against your opponent's.