to add to what urwumpe said, there are fusion bombs(H-bombs) as well as fission type devices, North Korea is not capable of producing a fusion based weapon yet.
Fusion devices wourk by having 2 stages, the first is a fission type nuclear bomb, this only does a small part of the explosive force its primary purpose is to compress a small amount of hydrogen gas(about as much as you'd need to fill a party balloon) until it fuzes to make helium. on and event by event basis this doesn't produce as much energy as a fission device but it does produce much more energy on a mass by mass basis( 1000 fusions produces less than 1000 fissions but 1kg of fusion fuel produces more energy than 1kg of fission fuel). there is no critical mass for fusion that can be achieved on earth(you need a ball of gas a few times the size of jupiter) so the bomb can be expanded to any arbitrary yield.
there is also a three stage bomb which can be even more powerful, a fission-fusion-fission bomb(the tsar bomba is believed to have been of this design) where the first two stages are as a typical hydrogen bomb but the tamper is made of U-238 which under the intense flux of neutrons from the fusion undergoes a massive amount of fission which adds even more enrgy to the explosion.
theoretically, there is no limit to the number of stages or fission-fusion cycles. but you'd be crazy to want to do this as multiple small warheads are much better than one large warhead.
