A shunt type welder is one that changes amperage by moving a core through a pair of secondary windings in the transformer. Any welder that has a crank or a large lever is probably a shunt type welder. They're often described as infinitely adjustable. The most common alternative is a welder that gets different fixed amperages from individual wires called taps which are connected to different locations on the windings of the transformer.
A tapped welder is one which changes amperages by clicking a lever on a dial with very little force, like a Lincoln buzz box, or has a series of outlets on the face of the welder marked with different amperage settings, and you plug your electrode holder cable into the hole with the amperage you want, like some old Forneys (and at least one old Lincoln model)is the most common alternative type of transformer welder. It's not infinitely adjustable because you can't (easily) go to any setting except those fixed settings. You can't dial in your amperage setting with perfect accuracy, but tapped welders don't go out of adjustment like shunt type welders.