A thin client may be an application program or a device for the execution of thin-client application programs.
In the former case, it is a program which communicates with an application server and does not incorporate the significant elements of business logic which the overall (client-server) application implements. Instead, the core functions of the application are located on a distinct computing device, an application server, which may be located nearby in a LAN or at a distance on a WAN or MAN.
In the latter case, it is a device which is designed to provide just those functions which are useful for user-interface programs. Often such devices do not include hard disk drives, which may become corrupted by the installation of misbehaved or incompatible software, but instead, 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, ROM, or a non-volatile memory device such as a Disk-On-Chip.