A pattern library is normally a collection of best UI practices: see the Yahoo UI pattern library http://developer.yahoo.com/ypatterns/ or http://ui-patterns.com/ for the two most well known examples. (It comes from Christopher Alexander's A Pattern Language and it being adapted to object orientated programming by the Gang of Four in 'Design Patterns: Elements of Reusable Object-Oriented Software'). The ALA version is a bit odd as it's more style-guide like (you normally have more options).