If people fight in irregular (not normal forces) armies not wearing a uniform, and not part of the official military (fighting group of a nation), they are called partisans if they fight against another nation's army, or terrorists, if they fight civilians.
Partisans and terrorists are less protected by the laws of war than soldiers.
Soldiers can be volunteers or conscripts (their nation calls upon them to fight).
Soldiers have existed for many years, dating back to ancient times.
Images for kids
Indian Army soldiers
fr:Militaire#Fonctions dans l'armée
Soldier Facts for Kids. Kiddle Encyclopedia.