When we talk about Nuclear Fusion, it is a nuclear reaction but it is very different from Fission reaction. In Fusion, two or more atomic nuclei collide at a much swift speed and as a result of that collision, they attach together to form a new sort of atomic nucleus. The most significant thing about this reaction is that during the process, matter is not conserved and there is a reason for that.
During the reaction process, some of the mass of the fusing nuclei does not stay the way they are. Instead, it is transformed to photons and is released over a cycle. The most common example of this process is our sun. The energy produced by sun is a result of countless number of nuclear fusion reactions on the sun that happen because of the collision of hydrogen nuclei and also due to the formation of helium nuclei.
The most important use of nuclear fusion is the contemporary world is for the energy purposes although such methods are restricted to a limited number of countries in the world. Why they are restricted to some countries? For one simple reason and that are the expenses. Carrying out a nuclear fusion process in a laboratory is not an easy thing to do. You need more than just laboratory and equipments. You need certified professionals and in addition to that, you need to have the money to spend on such experiments.
There is also a dark side of Nuclear Fusion. Like fission reaction, Fusion has its drawbacks as well. Nuclear Fusion can be used to create an atomic/nuclear bomb and it is considered to be more deadly than the nuclear bomb based on nuclear fission. These kinds of bombs are based on the isotope of Hydrogen atom and can cause more destruction than you can possibly imagine.