In a two-tier architecture the server holds both the application and the data. The application resides on the server rather than the client. The client can either be a thin client - dumb terminal with no processing power- or a fat client - hold at least some applications.
In two tier architecture the business logic layer is either clubbed with the presentation layer or with the persistence layer making either the client fat or the server fat respectively.
Refers to client/server architectures in which the user interface runs on the client and the database is stored on the server. The actual application logic can run on either the client or the server.
