Hearthstone is above all a strategy game and requires at least a little thought for you to beat your opponent. Which is why the decks you build should fit a well defined plan. It isn't possible to have what you need to be good at every style of the game in a single deck. You will have to set a strategy and adapt your cards to suit.
The aggro deck works well for a well established strategy, even if it isn't the most subtle way to reach your goal. The objective is clear, overcome your opponent as quickly as possible! To do this, the deck is built around cards that have a low mana cost to put pressure on your opponent from the start of the game. In the best case scenario, your opponent will be beaten in around the eighth turn.
The worst enemy of the aggro decks is obviously the taunt which slows down access to your opponents' health points and that you will absolutely have to get around! The area spells are also a calamity because they often overcome many of the little creatures. The outcome of the game for the aggro deck always relies on the first moments and your starting hand will also be decisive. So you'll need a bit of luck to set up a very aggressive game.
The aggro decks are often considered All-in, it works or it doesn't! There are no half measures and you will quickly know which way the game is likely to swing. Generally, if you haven't won the game by the 7/8th turn, the game is lost. There are many aggro decks, among them the Hunter Aggro Deck and the Warlock Murloc Deck are two of the classics.