Oscillation whose amplitude reduces with time is called damped oscillation as shown by graph 4. In graph 2, as amplitude is not changing. So, it is undamped oscillation. As the factor by which amplitude is changing increases. The particle looses its restoring nature and that oscillation called overdamped as shown by graph(1) and graph(3) is critically damped as its amplitude changes drastically and it is not actually a oscillatory motion.