A pure fission device (the technical nomenclature specifically avoids use of the term ‘bomb’ except in final, operation configuration such as the B-61 ‘gravity bomb’) creates a yield of energetic radiation, along with a small but unavoidable amount of wasted neutrons and radionuclides (residual daughter products from the nuclear material and activated components which themselves undergo a chain of radioactive decay) by causing a prompt fission reaction in [SUP]235[/SUP]U or [SUP]239[/SUP]Pu, which is contained and enhanced by a tamper.
A thermonuclear fusion device uses a fission ‘Primary’ to trigger the conditions for a fusion reaction (and often uses neutron yields from the Primary to breed tritium from [SUP]6[/SUP]Li as tritium is very expensive and has a short half-life of 12.3 years) to create a significant yield of energy from fusion which is primarily X-rays. This is also described as a “multistage” weapon because it could be built with additional stages of fission and fusion, but all deployed weapons are a fission Primary and fusion Secondary, with the yield of the Secondary sometimes used to significantly enhance the yield of the Primary by compressing a tamper made of [SUP]238[/SUP]U (or potentially other fertile materials; [SUP]232[/SUP]Th could be used for this, but in practice is not since there is plenty of [SUP]238[/SUP]U left over from uranium enrichment). While the energetic yield of pure fission weapons tops out at a few megatons of TNT equivalent, there is no theoretical limit to the yield of a nuclear fusion device. The US built and fielded a 25 MT device, while the Soviets tested the ‘Tsar Bomba’ which had an estimated yield of 52 MT but was incomplete in configuration, and had a projected yield in excesss of 100 MT in full configuration.
A boosted fission device injects a small amount of tritium into a fission reaction in order to get a small amount of “incomplete” fusion in order to further compress the fission Primary and get a large yield. It is essentially a less complex version of a fusion device with most of the yield coming from fission that is “boosted” by fusion.
A “neutron” device is basically a boosted fission device which is optimized to produce a large shower of neutrons instead of maximizing the electromagnetic radiation. It still produces radiation and blast effects, but of much smaller magnitude. (The thermal pulse and blast overpressure effects, as well as the characteristic mushroom cloud formed by the vacuum of superheated air rapidly rising are all due to the atmosphere absorbing X-rays from the device, and dictate the energetic yield in tons of TNT equivalency.). The effects of a neutron device are much more localized because of the modest blast effects and because neutrons don’t interact much with the atmosphere; instead, they penetrate deeply into solid materials and are absorbed, converting normally stable elements into highly radioactive isotopes.
As Chronos notes, one of the purported uses of neutron weapons is to turn the dense armor and ammunition used on modern tanks (made of depleted uranium, which is just [SUP]238[/SUP] left over from enrichment) into highly radioactive species, rendering them useless for protection. However, despite the reputation as being a tactical or urban weapon, the main use for neutron devices is actually in an anti-ballistic missile application where exposing opposing nuclear weapons to a high surge of penetrating neutrons, causing them to prematurely fission, poisoning the intended detonation schedule.
There are also in theory pure fusion weapons, in which the fusion weapon is initiated by some highly energetic non-fission reaction (e.g. an X-ray laser creating fusion-level temperatures and confinement, proton generators, or so-called ballotechnic explosives) but as far as the public is aware, none have every been developed or tested, much less fielded, and the energies necessary to initiate fusion without a fission primary have required massive capacitor banks or large arrays of the world’s most powerful lasers, so it seems unlikely that pure fusion weapons exist or will every be practical without some kind of major innovation in materials and energy technology.
While massive multi-megaton weapons are possible, they are energetically inefficient as beyond a certain size they simply result in pushing more atmosphere “up”. The atmosphere is opaque to X-rays over long distances and will limit the range over which atmospheric heating can occur. Early weapons, such as those on the LGM-25C, were massively overpowered to compensate for the low precision of the guidance system, but most operational weapons are in the <1 MT range, which is more than sufficient for hardened surface and unprotected urban targets, especially since such targets are typically assigned multiple weapons. There are also variable yield (‘Dial-A-Yield’) devices with a configurable yield option by varying the amount of boosting material injected, allowing use as a tactical-scale weapon with controllable range of effect. (The division of nuclear weapons into battlefield ‘tactical’ and international ‘strategic’ weapons is somewhere between a joke and a deception; any use of nuclear weapons between players with large nuclear arsenals is likely to become a strategic exchange just because there is no way to protect conventional forces from attack other than scattering them to reduce losses.)
‘Fallout’, by the way, is primarily due to material from the ground being sucked up into the mushroom cloud, activated by neutrons, and then dispersed by wind over a broad area. There is some of the device material and unconsumed nuclear material as well, but it is a modest amount compared to the many tons of debris which may be drawn up by the rising atmosphere. Fallout can be avoided by using higher altitude (>5 kft) detonation. Alternatively, fallout can be enhanced by near ground level bursts combined with a jacket of fertile material which the neutron output can convert to aggressive radionuclides, ‘salting’ the downwind real estate and making it uninhabitable and unpassible without protection for dozens or even hundreds of years.