A muzzleloader is any firearm into which the bullet is loaded from the muzzle of the gun. This can apply to anything from cannons to pistols, but in modern parlance the term most commonly applies to black powder small arms.
In general, the sequence of loading is to put in: a) gunpowder, either by pouring in loose powder, or by inserting a pre-measured bag of gunpowder usually called a "cartridge", b) wadding, soft material like cloth or paper, and c) the projectile, which may range from a solid ball to an assortment of scrap metal.
Since the items are generally tight-fitting, and the barrel long, a tool called a "rammer" is used to push the items down.
Muzzle-loaders must usually be cleaned ("swabbed") before reloading, so that there are no bits of burning material left in the barrel that may set off the next charge of gunpowder.