The five major components of a data communication system are a Message, a Sender, a Receiver, a Transmission Medium and Protocol.
The message is the information to be communicated. This can be anything from text and pictures to audio and video. A Sender is a device which sends the data messages. This can be anything from a computer or laptop, to a workstation or a mobile phone. A Receiver is pretty much the same, except it is receiving the information. Generally receivers can also send, but sometimes they clash in terms of compatibility due to differences in technology.
A Transmission Medium is the physical path by which a message travels from sender to receiver. This can be by wire, or wireless. It can be by coaxial cable, fibre optics, or by radio waves. The Protocol is the final piece. These are the rules that govern the data communications and transfers. It represents an agreement between the communicating devices, i.e. The agreement to allow the sender access to deliver to the receiver unchallenged. Text messages transfer unchallenged, but a Bluetooth connection requires confirmation.
Devices can be connected but not communication if protocol is not confirmed. An example of this is connecting a USB device. Most devices now are Plug and Play, meaning the computer confirms protocol and opens a gateway to the device in order to allow file transfer. However, in devices that aren't Plug and Play, these need software installed in order to install protocol and validate it upon connection.
Simply put, the general order of the relationship is that the Sender (or Transmitter) sends the Message via the Transmission Medium to the Receiver; all the while adhering to Protocol confirmed between the Sender and Receiver prior to the communication.