You'll find lots of info online about percussion (aka caplock) versus flintlock rifles/guns but basically .... A muzzle-loading gun (pistol or shoulder arm) is loaded by pouring gunpowder down the barrel and then putting a bullet (a lead ball or conical bullet) down the barrel and 'seating' it with a ramrod. To fire the gun the power has to be ignited somehow. Prior to percussion models there were flintlocks in which a piece of flint hits a striker plate to create a spark that ignites a small amount of power in the pan which sends the spark down a hole to the powder in the barrel. (Like in the movies "Last of the Mohicans" and "The Patriot"). The improved design was a brass cap (just like those red plastic ring caps in the toy department). The percussion cap went over a nipple and when struck by the hammer created a spark that went down the hole to the powder. The flintlock took longer and much more care to load -- you have to pour powder into the little pan -- and was very unreliable in wet weather. With the percussion cap, you could put in on the nipple with the hammer on it. Then when you were ready to fire, you just cock the hammer. With a revolver, you would put a cap on the nipple of each chamber and you're ready to fire six rounds in succession (like the movie "Outlaw Josey Wales"). With modern ammo -- invented before the Civil War but not in general use until later -- everything is put together, i.e. the primer is in the base of a metal cartridge filled with power with a bullet on the end. The concept is the same from ancient matchlock guns to modern guns.

And you many not know a "rifle" means there is "rifling" in the barrel. Rifling is spiral grooves inside the barrel that make the bullet spin giving it much more accuracy. With the exception of shotguns, virtually all modern guns are rifled. At the time of the Civil War, some smoothbore guns were still in use and there were still some flintlocks in use -- especially when men brought their dad's old gun when they enlisted. At the beginning of the war, there were many 'conversions' in armories around the country -- old guns that had been converted from flintlock to caplock.