A tubular magazine usually runs under the barrel allowing rounds to be loaded tip to primer. This works well for shotgun shells, because there is not a pointed protrusion that can trigger the primer on a shell. The magazine load is limited by design particularly on shotguns because of waterfowl laws and other local, state and federal laws. Usually you can load four in the tube and have one chambered for a total of five. However, some believe that leaving five rounds loaded for extended periods may wear out the magazine’s spring, so some only leave three in the tube to reduce stress. This is only conjecture however, but it does appear that springs can wear out from use and not just because they are compressed for long periods.
Various semi-automatic shotguns can have a detachable magazine. This typically means the rounds are loaded into a box type magazine or clip and is then inserted into the shotgun just as if you would a rifle or even a handgun. The magazine is loaded then inserted versus loading the under the barrel tube magazine.