Fig.2 shows the carrier being switched on for a short time to produce a pulse of R.F.

This is the principle of Radar; a short pulse is transmitted and then an echo listened for.
Fig.3 shows a long pulse and three short ones.

This generates the letter B in Morse Code.
Fig.4 shows Pulse Width Modulation (PWM).

The width of the pulse is determined by the amplitude of the modulating signal at that instant.
Fig. 5 shows Pulse Position Modulation (PPM).

Here the width and amplitude of the pulse are constant but its position is determined by the amplitude of the modulating signal.
PULSE CODE MODULATION is where the amplitude of the modulation is measured at regular intervals and a binary number generated to represent that amplitude.