xauth is extended authentication in vpn. this is also called as 1.5 IKE, this comes after phase 1 and
before phase 2.
in vpn's pre share key is one layer of security to authenticate the correct user, the second layer is xauth. so if someone needs to breach your network using vpn he will need to breach 2 walls.
and as you said this terminology is used in VPN
you can read more in the following link
http://www.cisco.com/en/US/docs/ios/12_1t/12_1t1/feature/guide/xauth.html
u auth means user authentication, it is just a concept of authenticating users to control access
i think i would be right to say xauth uses u auth : )