The SOLID principles are a collection of concepts originally put together by Robert C. Martin, aka "Uncle Bob." Usually advertised as Object-Oriented Principles, you should think of them as just plain good architectural choices. The SOLID principles consist of five principles: the Single Responsibility Principle, the Open/Closed principle, the Liskov Substitution principle, the Interface Segregation Principle, and the Dependency Inversion Principle.

The original articles on the SOLID principles are available at http://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod.