The North Koreans have offered few details of their missiles. Satellite photos apparently reveal that these missiles look like they were developed from SCUDs, as well as some earlier Chinese ballistic missiles. The Chinese deny that they have cooperated with the North Koreans, but the Chinese has regularly denied things like this, and later found to be lying.
The North Korean missiles are not high tech. They have taken the Russian approach of using simple technology and then scaling it up. This has worked for the Russians, and many American engineers have urged that the United States adopt this less expensive practice for satellite launching. But these missiles, using liquid fuel, take hours to ready for launch. Military missiles, ideally, should be made so they can be launched quickly. This is why American missiles use more expensive solid fuel and construction that allows rapid launching. The North Koreans also have less accurate guidance systems, and building guidance system for longer range missiles is more difficult. But if they use nuclear warheads, accuracy isn't critical if you are just firing the missiles at large cities.
The North Korean missile program operates on a shoestring. A large chunk of the North Korean engineering and industrial capacity has been applied to their missile program. For a small, poor country, they have done a lot with very little. But they have not demonstrated that they have solved all the reliability and accuracy problems necessary for creating reliable long range missiles. They are getting close, but that's not the same as getting there. Indicative of those problems the North Koreans declared a moratorium on missile tests in late 1999. But they have been caught testing new engines on the ground since then, and are obviously still working on missile technology.
North Korean Ballistic Missiles
SCUD B has a range of up to 340 kilometers, weighed 5.9 tons and has a one ton warhead.
SCUD C has a range of up to 550 kilometers, weighs 6.4 tons and has and a .6 ton warhead.
Nodong 1 (also known as the SCUD D ) has a range of up to 1,300 kilometers, weighs nearly ten tons and has and a .8 ton warhead.
Taepodong 1 (also known as Nodong 2) has a range of up to 2,000 kilometers, weighs over 25 tons and has and a one ton warhead. This was a two stage rocket, a tricky bit of engineering.
Taepodong 2 (Nodong 3) has a range of at least 6,000 kilometers, weighs at least 85 tons and has a one ton warhead. This is a three stage rocket. Not yet flight tested.
The North Korean army has 500 SCUDs, of different models, in service.
North Korea, a small, poverty stricken police state, has shocked the world by developing long range missiles. This should not be surprising, as North Korea has been at it for thirty years and has kept their design goals simple. In the early 1970s, North Korea got access to rocket building technology from China. The North Korean built their own versions of Soviet designed FROG rockets. But neither China nor Russia would sell North Korea any ballistic missiles. So in 1976, North Korea signed a military treaty with Egypt and received SCUD B missiles and launchers as part of that deal. By 1984, the North Koreans were testing an improved (by them) version of the SCUD B. In 1986 the North Korean SCUD C was tested and in 1988, the SCUD B was offered for sale. At that point, North Korea was manufacturing a hundred SCUD B's a year, and was selling most of them for a million (or more) dollars each in the Middle East. In 1989, the SCUD C development was finished. But research and development continued and the "SCUD D" appeared instead as the Nodong I. This missile was sometimes confused with the Soviet SCUD D, which was a different approach to modifying the SCUD. The Nodong I missile was test fired in 1993, at which time it became known that the North Koreans were developing a "two stage SCUD" that they were calling the Nodong 2 or Taepodong 1. This was a major advance. Simply making SCUDs longer (to carry more fuel) and tweaking the engine and guidance system to extend the range was one thing. A two stage rocket was a major advance. The larger first stage was apparently assembled using several Nodong engines working together, with the second stage being essentially a single Nodong. The first test of the Taepodong 1 was in 1998.