A client is a member of a class of programs in a modern software architecture paradigm called client/server software architecture, in which all applications have two components: the client component and the server component. The client component typically has the following characteristics:

• A client is executed on-demand; only when there is a need for it (contrasted with a server which usually is always executing).
• A client is active; it initiates all dialogue to a server as needed to perform its tasks. (A server is passive and waits for requests, never initiating any communications with a client).
• A client interfaces with humans (whereas a server normally runs in the background and interfaces only with other software (clients)).
• At any given time there probably numerous instances of the client component of the application running on different computers throughout the network, all of which are connecting to only one or two servers (a server typically is a centralized process which serves many clients).