An executing program, which is the one that has accepted control from the client terminal,
starts to take some actions according to its inherent algorithm.
The program contains lines of code,
which determine the order of program execution.
The general order of program execution consists in sequential transfer of control from one line to another in the top-down direction.
In following sections,
we will consider what can be written in program lines of code, and according to what rules it can be written.