Numerous applications run in a client/server environment, this means that client computers (computers forming part of the network) contact a server, generally a very powerful computer in terms of input/output, which provides services to the client computers. These services are programmes which provide data such as the time, files, a connection, etc.
The services are used by programs client programs which run on client computers. This is why the term "client" is applied (FTP client, email client, etc.), where a program is designed to run on a client computer, capable of processing data received from a server (in the case of the FTP client we are dealing with files whereas for the email client we deal with email email).