Client Server Applications: The definition

What is Client Server Architecture ? What is some of the defining characteristics of Client Server Applications, especially since there is a buzz for the past many years about web applications, or a mixed breed (using applications such as Adobe AIR).
Client Server applications are literally define by the names used where one software application, at the client end (or at the user end) makes a service request to another software application that sits at the server (typically a machine with a much higher configuration). However, the separation between the client and server is logical, since both of them could exist on the same machine. There is a process to separate the work load of the application between the server application, and the client application. Client server applications are one of the central concepts behind network computing. Initially, the term was used to differentiate between the mainframe model or the Unix model where the entire work was done at the server, and the client was typically dumb, with no capability to do any processing.