Thin Client refers to a terminal or a computer connected to a computer and has no additional accessories other than a processor and a hard disk. Thin clients do not contain any CDROM drive, floppy disks etc. The main purpose of them is to act as a client and not as a server, and all requirements are met through network drives on the server that allow each client to access the other accessories like CDROM drives.
Thin client is also the concept behind NetPC and Network Computer though they are different in implementations. NetPC is from Windows and Intel and Network computer is a concept backed by Sun Microsystems and Oracle.