A thin client (sometimes also called a lean client) is a client computer or client software in client-server architecture networks which depends primarily on the central server for processing activities, and mainly focuses on conveying input and output between the user and the remote server. It is usually a a network computer without a user writable long term storage device, which, in client/server applications, is designed to be especially small so that the bulk of the data processing occurs on the server. The embedded OS in a thin client is stored in a "flash drive", in a Disk on Module (DOM), or is downloaded over the network at boot-up.
Thin client computing is a server-centric computing model in which the application software, data, and CPU power resides on a network server rather than on the client computer. The client and the server communicate each other using certain protocols designed solely for thin client computing.
Example Of Thin Client Systems
Some of the most common thin client systems are:
VNC: Virtual Network Computing
Virtual Network Computing (VNC) is a graphical desktop sharing system which uses the RFB protocol to remotely control another computer. It transmits the keyboard and mouse events from one computer to another, relaying the graphical screen updates back in the other direction, over a network. VNC is platform-independent — a VNC viewer on any operating system usually connects to a VNC server on any other operating system.
Windows Terminal Services
. Terminal Services is Microsoft's implementation of thin-client terminal server computing. With terminal services, only the user interface of an application is presented at the client. Any input to the application like mouse or keyboard events, is redirected over the network to the server, where all application processing takes place. The client uses the Remote Desktop Protocol to communicate with the terminal server.
Novatium’s Net PC
Novatium’s Nova NetPC  is an example of a thin client, designed to connect to any server (Windows/Linux/Solaris) remotely and provide the user with full capability desktop. It is developed by the TeNet Group  of IIT Madras and Novatium. NetPC runs on a mobile phone’s processor and costs around $100. The users can login into server from home and run applications or access data. Users are free from botheration of computer viruses, installation or up gradation of applications.