Aggro-deck is a term borrowed from games like Magic the Gathering. It means a deck that intends to play aggressively and kill your opponent as quickly as possible.
An aggro deck in Hearthstone is characterized by the following:
- Low mana curve (allows you to play many cards quickly)
- Aggressive creatures (high power relative to toughness) with abilities like charge, battlecry, overload and stealth
- Direct damage (burn) spells
Forcing the opponent to attack your minions with taunt rather than your hero is not a primary characteristic of aggro decks, because an aggro deck will not worry about taking damage since it is going to kill you before you kill them. An aggro deck will typically avoid playing taunt cards because the player is choosing to sacrifice defensive power for offensive strength.
Some cards that would fit the classic "aggro deck" include lava burst, reckless rocketeer, unleash the hounds, and nightblade. These cards fit because they focus on dealing as much damage to the opponent as quickly as possible, and have an effect the turn they enter the battlefield.
The opposite of an aggro deck is a control deck, which is a deck focused on removal and counters, defenses, cards that get stronger over time, and keeping a full hand of cards.
Cards that are neither aggro nor control can be referred to as "midrange" decks. Those focus on cards that are extremely efficient.