The first component of X is the server. It creates the windows, draw images and text, in response to programs. The second basic component of X are the client programs. The application programs make use of the windowing facilities. They ask the server to perform particular tasks.
Client programs do, in general they perform other tasks, e.g. editing text, drawing diagrams.
The third basic component of X is the communications channel. Clients use it to send requests to the server and the server uses it to send back status and other information to the client programs. The clients are running on one machine, but the display and its server is running on another.