Diagram of a W87 nuclear warhead, showing its variation of the Teller-Ulam design. On the top is a spherical secondary, while at the bottom is a spherical primary. 
The outer casing (blue/purple) is that of a MIRV warhead reentry vehicle (very schematic) which would protect the warhead from the heat of the atmosphere after its launch into space. The "physics package" of the primary and secondary is surrounded by a uranium-238 reflector casing (gray), inside of which is filled with polystyrene foam (tan) which aids in the transfer of radiation from the primary to compress the secondary. The primary (bottom) is a tamper of some sort (gray), followed by precisely arranged high explosives (yellow), followed by a plutonium core (blue), filled with tritium gas (green). The spherical secondary is a uranium-235 or uranium-238 tamper (blue), followed by lithium deuteride fusion fuel (red), followed by a uranium-235 or plutonium "sparkplug" (blue). 
This is not drawn with any precision or scale and are meant only to illustrate a general concept. 

Based primarily on [1] published in U. S. News and World Report in 1995 and reprinted in the 1999 Cox Report. More images available online at Howard Morland's "The Holocaust Bomb: A Question of Time".
Own work
Fastfission, HowardMorland