A war is when two groups of people struggle for power, using weapons. A war between two groups of people in the same country is called a civill war.
Wars cause injury and death to the armed forces and also to civilians (people who are not fighting) and many people have their homes destroyed or they flee to another country, becoming refugees. There are millions of refugees in the world today.
Wars have happened throughout human history and there are many wars in the world today. It is unlikely that we will ever see the end of wars.