WPA-PSK

Wi-Fi Protected Access with Pre-Shared Key is a special mode of WPA for users without an enterprise authentication server. Instead, for authentication, a Pre-Shared Key is used. The PSK is a shared secret (passphrase) that must be entered in both the AP or router and the WPA clients. 
This pre-shared key should be a random sequence of characters at least 20 characters long or hexadecimal digits (numbers 0-9 and letters A-F) at least 24 hexadecimal digits long. After the initial shared secret, the TKIP (Temporal Key Integrity Protocol) handles the encryption and automatic re-keying.