There are two main categories of nuclear weapons, classified by their mechanism of operation: fission bombs, which break apart heavy atomic nuclei to release energy, and fusion bombs, which fuse light nuclei. Fusion bombs tend to be much more powerful. Within these nuclear weapon categories, there are slight variants: for example, a salted bomb is surrounded by a layer of material that can become highly radioactive with neutron bombardment, and fission-boosted weapons are nuclear weapons that, despite being based on fission, exploit fusion reactions to boost their yield. Neutron bombs, or enhanced radiation weapons, are fusion weapons designed to emit intense neutron radiation, killing all life within a certain area but doing less damage to buildings.
Most nuclear weapon variants are designed for the purpose of having a spectrum of available yields and sizes for different applications. The most fearsome nuclear weapon of all time was the Tsar Bomba, a Soviet fusion bomb with the explosive force of 50 megatons of TNT. At first it was designed to have a yield of 100 megatons, but this was scaled down due to fallout concerns. In contrast, the smallest nuclear weapons, like some tested for Operation Plumbbob at the Nevada Test Site, may have a yield as low as a mere ton of TNT, or less. The smallest nuclear weapon mass-produced for deployment was the Davy Crockett warhead, designed for infantry launch from small redeployable mortars. It was deployed in Germany to guard against a Soviet invasion of Europe.
The earliest nuclear weapon designs were modeled after small guns, which shoot a hemisphere of highly-enriched uranium into another hemisphere of the same, kickstarting a nuclear reaction and the ensuing release of heat and light in large quantities. More modern designs use implosion assemblies, where spheres of segmented uranium are surrounded by chemical explosives that all detonate simultaneously, concentrating the uranium in the center and starting a chain reaction.
It is possible to make nuclear weapons that are quite small, on the order of size of a toaster. Because extremely large nuclear weapons cause collateral damage in the form of fallout, and have reduced yields because more of the uranium is blown apart without fissioning, the military favors nuclear weapons in the small-to-medium range. More focus in put on the method of delivery. Until they were decommissioned in 2005, the most fearsome delivery method for nuclear weapons worldwide was the American LGM-118A Peacekeeper missile. It contained 10 reentry vehicles, each with a nuclear warhead 25 times more powerful than the bomb that incinerated Hiroshima. One of these could dole out destruction across a very wide swath of land.