Protocol

Protocol is an agree-upon format used for exchange of data between the two computer machines/devices.

The protocols determine the following.

Data compression method
The type of error checking to be used. 
How the receiving device will indicate that it has received a message.
How the sending device will determine that it has finished sending a message.

There are different standard protocols being applied by the programmers. Every protocol has its advantages as well as disadvantages. For instance, few are simple to use than the other ones. Some are faster and some are much more reliable.