An Aggro deck (Agressive Deck) is one of the three basic deck types. This deck type aims to win by dealing as much damage as possible as quickly as possible, normally through the use of creatures and burn spells (spells that deal direct damage to a creature/player).
Aggro decks have three main elements: curve, threat density, and damage output.
Threat density is the number of threats a deck puts out. Since an aggro deck relies on dealing as much damage as possible, players aim to play as many threats as possible per turn.
A low mana curve allows the player to play multiple threats per turn (e.g., a one-drop on turn one, a two-drop on turn two, a one-drop and a two drop on turn three, etc.) in the hopes that the opponent will not be able to muster up enough answers in time.
Damage output is how much damage a card can do. At first glance, a Lightning Bolt seems to do more damage than a Savannah Lions, but in theory the Savannah Lions can do more damage: The Lightning Bolt will only be used once, but since the Savannah Lions is a permanent, it will be able to attack for each turn it is in play. After two attacks, the Savannah Lions has already done more damage than the Lightning Bolt.
Of course, a creature with a higher damage output is better suited in a deck than a creature with lower damage output. Damage output is calculated by multiplying the creature's power by the number of times it will attack. Because of Wrath of God, damage output for creatures should only be calculated up to turn four. For example, a 1/1 creature played on turn one will do 3 damage; a 2/2 played on turn two will do 4 damage; a 3/3 played on turn three will do 3 damage; and a 4/4 played on turn four won't do any damage unless it has haste.
Some good cards to play in an aggro deck include Fleecemane Lion, Bloodsoaked Champion, Soldier of the Pantheon, and Firedrinker Satyr. In an aggro deck, you cannot stumble on mana. Good lands to play in an aggro deck include Urborg, Tomb of Yawgmoth, Mana Confluence, and other lands that produce multiple colors of mana. In an aggro deck, your life total should only be just another resource. Don't shy away from playing cards like Ulcerate or Caves of Koilos. Because, if played right, an aggro deck should deal more damage to the opponent than its controller is taking.
The Bane of Aggro
The number one bane of aggro is control. With Sphinx's Revelation, Wrath of God, and Supreme Verdict, you aren't going to gain much leverage in the late game. The best solution I've found is to make it so your deck has two modes: Aggro and Midrange. The way you can do this is by making your sideboard have specific cards to swap out and make your deck oppose any threats. You're going to want to put three or four nonbasic lands in your sideboard, since most aggro decks only run 20-22 lands. One key card to board in against control decks is Sylvan Caryatid. At only two mana, you can play it early in the game. The fact that it has Defender and Hexproof make it difficult for Control decks to deal with. Control decks tend to do quite a bit of destroying, so having a couple Scuttling Doom Engine in your sideboard couldn't hurt. Elspeth, Sun's Champion is good against burn decks and beefy creatures, so she is a fantastic addition to your sideboard. It is also useful to have Drown in Sorrow against the mirror matchup. Bile Blight is also amazing against token decks and enemy Elspeths. Last Breath is also a handy card to have in case your life total is seriously threatened. Cast one on your Bloodsoaked Champion, and just recast him next turn. In short, be sure to have answers if your opponent pulls out Traumatize and mills you for twenty life.