Operating System - Unix and Linux are quite popular server operating systems. Microsoft's Windows NT is capable of becoming a server as well. A client is any other program asking for information from a server. An example of a client is your PC at home. Clients request information from servers.
Servers are usually more powerful and have large storage capacity to serve many clients at a time.
To store information on one machine instead of keeping it on all the clients machines. This means that if information changes, the change can be made in the server and appear in all the clients without clients having to reconfigure their machines. It also allows clients to be made smaller and simpler, since a good deal of the resources are provided by the server. A mirror is a server that looks like and serves an identicial piece of information to a client. These servers act the same but are at different URLs.