A regular 'atomic' bomb is a fission reaction. A hydrogen bomb uses a fission bomb to cause a nuclear fusion reaction. H-bomb = much more powerful.
Most of the energy from large H-bombs still comes from fission.
You can surround the fusion stage with natural uranium. While natural uranium has to much U-238 to undergo a runaway chain reaction, the fission reaction emits enough high energy neutrons to induce a significant fission reaction.
This link seems to work: http://www.bloomberg.com/news/articles/2016-01-06/north-kore...
Here's a New York Times report: http://www.nytimes.com/2016/01/06/world/asia/north-korean-tr...
No real detail, though. Artificial earthquake detected. North Korea apparently claims a successful hydrogen bomb test.
You may enjoy this: http://nuclearweaponarchive.org/Library/Teller.html
I'm not well schooled in nuclear bombs. What's the payload, how does this compare to their previous tests, and what's next?