Projectile weapons deal damage at a distance, but use ammunition to do so.
Guns use bullets for ammunition. They typically deal more damage than an equivalent bow. Currently guns use all the ammunition equipped, with no need to reload. Because of this it is easy to go through ammuntion rapidly.
Bows use arrows for ammunition. Currently there is no mechanism to draw the bow, it fires in a relatively straight line, like a gun. There is only one type of arrow, made using 1 wood for every 4 arrows.