It can be difficult to describe rich internet apps (whether they be javascript or flash based) to a client. This article on infoworld has a pretty good definition:

RIAs attempt to combine the strengths of desktop and Web applications without falling prey to their weaknesses. RIAs try to present most of their user interfaces at the client so that they can be responsive and the interface can be as complex as it needs to be. RIAs often do need an installation, but usually only for the runtime engine, which tends to be small and most often updates itself automatically. The RIA application itself typically executes on the remote server.

Application development: The new face of Internet apps

Many companies and organizations have traditional web applications and users are confortable with them. It's important to make easy-of-use and user experience an overridding goal with any Rich Internet Application project.