Orchard extensibility explained - aka why Orchard's flexible content model rocks
Bertrand Le Roy's not-so-recent article "Orchard Extensibility" in MSDN Magazine explains the various aspects of - surprise - extending Orchard. The article mainly covers Orchard's content model, its concepts and flexibility: something we all love in Orchard. Bertrand also lists some other important parts of the Orchard API, all worth knowing.