Role of 'Into' in a LINQ query

var query = 
from u in dc.UserDetails  join s in dc.SchoolDetails
                on u.SchoolId equals s.SchoolId <big>into </big>JoinedUserSchool
                            from s in JoinedUserSchool.DefaultIfEmpty()
                            select new
                                {
                           UserName = u.FirstName,
                           SchoolName = s != null ? s.Name : null
                                };