Brad Plumer has this telling chart
over how the economies of North Korea and South Korea has developed over the last few decades:
The main reason for this divergence is that North Korea is a planned economy while South Korea's is mostly market based, but the North's extreme trade isolationism/protectionism and it's extreme militarism (using more than 25% of its GDP on its military and with more than a fifth of men aged 17-54 in active military duty with the rest being considered reserve soldiers) have also contributed to this divergence.
The end result of this is not just that they're generally poor in North Korea, but also that millions have starved to death and much of the rest are malnourished (resulting in the fact that
they are on average 15 centimeters (6 inches) shorter than in South Korea
) and the fact that they don't have any electricity much of the day or during nights, as can be seen in this classic photo: