Deltanine

Not content with the destructive force of the atomic bombs dropped n Japan at the end of World War 2 Teller set out to design a bomb over 3 orders of magnitude (over 1000 times) more powerful. The result was the Teller-Ulam configuration of which most, if not all, thermonuclear weapons are based today.
It's hard to comprehend the fact that the bomb dropped on Hiroshima nearly 10,000 times smaller than the later bombs made possible by Teller's genius. The largest bomb ever detonated had the energy equal to 1% of the sun's output, and if dropped on Berlin would have been visible in London.

80% of the energy given off in a fission bomb is in the form of x-rays. In a Tell-Ulam bomb these x-rays are reflected around inside the casing and used to compress the hydrogen fuel. It can do this because the x-rays travel at the speed of light - an eternity compared with the shock wave that will eventually destroy the bomb casing. In fact although the fission stage takes about 550 billionths of a second to detonate the second hydrogen stage takes between 20 and 50 billionths of a second.

A really good flash animation of how this all happens can be found here.