The process we are going to use requires the aircrack-ng suite so before we start one needs to download and install the "aircrack-ng" package on their Linux box. I believe that it is also available for Windows but is more complex to get up and running as most windows based apps.
The weakest option is to use WEP encryption. This is not advisable as one can crack it in less than 5 minutes or so. Here is the process explained in detail:
The important point we learn from this is that the password complexity is not very relevant in the WEP encrypted network as the difference between cracking a 10 char hexadecimal key (64 bit encryption) and the 26 char hexadecimal key (128 bit encryption) is only a couple of extra minutes of packet sniffing.
Now if one is wise to use WPA / WPA2 encryption the process is similar with the difference that one cannot associate to the AP and use injection to get the IVs. Therefore we adapt our approach as follows:
A dictionary attack can last a long time and result in failure so I would recommend that you spend a fair amount of time preparing a good dictionary based on any information about the access point owner and also using an alphanumeric password file with common character substitutions. The use of John the Ripper in Linux or Cain in Windows is recommended!
A complex WPA password of 11-14 characters in length that is not a common dictionary word and contains small letters, capital letters, numbers and special characters will take over a week to decrypt using the most powerful processors available and this will probably not be pursued by the average attacker. The important thing to note is that in WPA encryption password complexity plays the key role in securing the network!
Once in the network a malicious outsider can sniff all your traffic including personal login credentials, bank details from forms, browsing activity and many other info exposing the network to complete identity theft and information leakage. So please be aware, use the right encryption and password complexity and secure your networks.