Data Layer, Business Logic Layer, User Interface Layer. Job done! Well, not quite. N-Tier is a vague term at best and doesn't really say anything more than pointing to the idea that there should be some kind of separation of concerns between various chunks of code. Nowadays describing the architecture can be harder than building it - SOA, explosion of the cloud services and storage, Facebook as front-end, NoSQL databases and many more. This would be open session for open minds in a form of a practical discussion for real life problems and possible approaches and solutions.