I wrote this when I was the Digital Library Applications Lead at the University of Notre Dame. It was archived from http://www3.nd.edu/~dbrubak1/ which no longer exists.
Standards are what we say we will do.
Standards are written by committees. Standards are enforced by policies. A standard without a mechanism for policy enforcement is an empty promise.
Practices are what actually happen.
Practices arise out of patterns of behavior created while people do their work. Practices are enforced by social contracts. Practices diverge organically without clear leadership.