An application program communicating with an application server and relies for most significant elements of its business logic on a separate piece of software, an application server, typically running on a host computer located nearby in a LAN or at a distance on a WAN or MAN.
A thin client does most of its processing on a central server with as little hardware and software as possible at the user's location, and as much as possible at some centralized managed site.
A thin client as a device is designed to provide just those functions which are useful for user-interface programs. In the interests of low maintenance cost and increased mean-time between failures (MTBF) the thin client device will use read-only storage such as a CD-ROM or flash memory.