North Korea claims it tested a hydrogen bomb on January 6, 2016, but it probably isn't true. For starters, the seismic disturbance caused by the explosion was a magnitude 5.1, according to the U.S. Geological survey. That's similar in strength to the disturbance caused by its atomic bomb test recorded in 2013.
Hydrogen bombs are many times stronger than atomic bombs. This insightful Discovery News video explains the science behind both weapons and how they differ.