I just listened to this podcast from www.hanselminutes.comwhere Scott Hanselman interviews “Uncle Bob” (Robert C. Martin) and thy talk about SOLID Principles
Single Responsibility (SRP)
Open Closed Principle (OCP)
Liskov Substitution Principle (LSP)
Interface Segregation Principle (ISP)
Dependency Inversion Principle (DIP)
Find more here: http://www.hanselminutes.com/default.aspx?showID=163
and listen to the podcast, you can play it in the browser using the embedded flash player