Nuclear weapons are those explosive devices which derive the destructive force in them from nuclear reactions. Nuclear reaction is a process in which two nuclei (i.e. nucleus of two atoms or the nucleus of an atom and a subatomic particle from outside the atom) collide and produce one or more nuclides.
These nuclides produced differ from the nucleus that began the process. When such a nuclear reaction happens (either fission or fusion) large amounts of energy is produced in the process which is destructive in nature. Thus it is right to state that the nuclear weapons use nuclear reactions to emit huge amounts of destructive energy.
All the countries around the globe have developed some sort of nuclear weapon for protection etc other purposes. But irrespective, here is a brief about the countries with the highest nuclear weapons. Russia tops the list followed by USA with formers total arsenal count being 10,000 of which 2430 are operational and strategic. The arsenal count for USA is 8500 with 1950 of them being operational and strategic.
France, China and UK follow with the total arsenal count being 300 (290 operational and strategic), 240 (none operational) and 225 (166 operational and strategic) respectively. Pakistan, India, Israel and North Korea also make the list 90- 100, 80- 100, 80 and 10 (respectively) being the arsenal count, with none being operational and strategic.