Just got the ".NET Framework 4.5 Expert Programming Cookbook" - going through the first example of implementing the Repository pattern. I've been around code for a long time (14+ years) but there's some new syntax that I'm not familiar with:

        public bool IsUsernameUnique(string userName)
            bool exists = _users.Exists(u => u.UserName == userName);
            return !exists;
I'm not getting this part:

u => u.UserName == userName

_users is a List<User> collection.

Is this Linq ?? What's the u => ?