WEP | WPA PSK

The protocol IEEE 802.11 using WEP is very weak.

The three core deficiencies with Wired Equivalent Privacy (WEP) are the use of static encryption keys (Rivest Cipher 4 {RC4} algorithm, which is a stream-symmetric cipher), the ineffective use of initialization vectors (the same IV values are used over and over again), and the lack of packet integrity Assurance (flipping specific bits and altering the Integrity Check Value). 

Also briefly discuss WPA PSK mode. Is it hackable?

1. Wi-Fi Protected Access (WPA) was an intermediate measure to take the place of WEP, pending the availability of the full IEEE 802.11i standard. WPA2 replaced WPA and implements the mandatory elements of IEEE 802.11i.  If authentication server (AP) like Remote Authentication Dial-In User Service (RADIUS), is used its WPA-Enterprise.  If pre-shared key is used in WPA, its Wi-Fi Protected Access pre-shared key (WPA PSK). A user cannot make network call unless he is fully authenticated ( true only in WPA-Enterprise, not in PSK), Further at lower layers, it uses Temporal Key Integrity Protocol (TKIP) [RC4] and Counter Mode Cipher Block Chaining Message Authentication Code Protocol, Counter Mode CBC-MAC Protocol (CCMP) [AES algorithm].  AES (under WPA2) is a more appropriate algorithm for wireless than RC4 (under WPA). 
2. Cracking WPA2-PSK Passwords with Cowpatty & Aircrack-Ng