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. A conscript is a person who has been forced to become a soldier and fight. In countries such as Switzerland, Israel, and Finland, men are conscripted for a few months once they become legal adults.
There are many different kinds of soldiers, and the term to describe them changes depending on their job and part of the military they serve in. For example, soldiers that serve in the navy can be called sailors or seamen.