Service Work performed (or offered) by a server. This may mean simply serving simple requests for data to be sent or stored (as with e-mail servers, file servers (File Transfer Protocol), HTTP or Gopher servers, SQL servers, etc.); or it may be more complex work, such as that of IRC servers, print servers, X Windows servers, or process servers. — FOLDOC: Free Online Dictionary of Computing
Client-Server is a common form of distributed system in which software is split between server tasks and client tasks. A client sends requests to a server, according to some protocol, asking for information or action, and the server responds.
This is analogous to a customer (client) who sends an order (request) on an order form to a supplier (server) who despatches the goods and an invoice (response). The order form and invoice are part of the “protocol” used to communicate in this case. — FOLDOC
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