The term "Rich Client" refers to the concept of rich and interactive user interfaces that are locally deployed to the client machine, as opposed to "Thin Client" which refers to Web-based applications.
Since the late 90's Web-based applications have gained wide adoption, primarily because of the ease with which they could be deployed to end users (on any Internet-connected PC). However, Web-based interfaces are not conducive to highly interactive application interfaces. Writing complex client-side script (that is browser-neutral) is not a viable approach to increase interactive features. The business community has realized for some time now that hosting Web-based solutions is no silver bullet to reaching their user base. For one it requires technical know-how to host a 24x7 operation, and the business infrastructure to support it. Furthermore, offline work is not possible from a Web-based application.
The new generation of rich client is known as the "Smart Client". It brings back the rich user interface that we traditionally had with client/server applications in the early-mid 90's, with a twist. Rich interactive interfaces provide enhanced usability and productivity for end-users. This is achieved with standardized interfaces using typical interface components such as menus, context menus, toolbars, hierarchical data views, data grid tables, charts and any number of others that make it easy to interact with the data being collected or presented. Further to this, the smart client adds Web-based deployment with the ease of Web-based applications today, with additional support for automatic version updates, online/offline functionality and methods for managing client-side security requirements in a controlled and safe manner.