A client is an application or a system that accesses a remote service on another computer system, known as a server, by way of a network. The term concerns devices that are not capable of running their own stand-alone programs, but could interact with remote computers via a network.
The client–server model is still used today. Client and server can run on the same machine and connect via UNIX domain sockets. Using Internet sockets a user may connect to a service operating on a possibly remote system through the Internet protocol suite.
Mail Clients are example for the access to remote service on another computer system. Mail clients access a server in order to send and receive text messages (E-Mails). To retrieve their email the clients don’t need to be logged in permanent – their internet service provider retrieves and stores the mails.