This short tutorial walks you through hacking and cracking WPA/WPA2 based networks which use pre-shared keys.
WPA/WPA2 supports many types of authentication beyond pre-shared keys. aircrack-ng can by now only crack pre-shared keys. So ensure airodump-ng shows the network as having the authentication type of PSK.
If you know howto hack or crack a WEP based network, you might know that the WEP cracking and hacking methods are based on statistical methods. This WPA/WPA2 is unlike the WEP hacking / cracking approach based on plain brute force techniques. The brute force method is rather slow, since the pre-shared key can be from 8 to 63 characters in length, which might require too much time to crack.
UPDATE Oct 12 2008 A new project called Pyrit is currently under it's way. "Pyrit takes a step ahead in attacking WPA-PSK and WPA2-PSK, the protocol that today de-facto protects public WIFI-airspace. The project's goal is to estimate the real-world security provided by these protocols. Pyrit does not provide binary files or wordlists and does not encourage anyone to participate or engage in any harmful activity. This is a research project, not a cracking tool.
Pyrit's implementation allows to create massive databases, pre-computing part of the WPA/WPA2-PSK authentication phase in a space-time-tradeoff. The performance gain for real-world-attacks is in the range of three orders of magnitude which urges for re-consideration of the protocol's security. Exploiting the computational power of GPUs, this is currently by far the most powerful attack against one of the world's most used security-protocols."