The ham and cheese is the sandwich every other sandwich is measured against, and it earns that role by having nowhere to hide. Two slices of soft white or wheat bread, a few slices of cold sliced ham, a slice or two of cheese, a swipe of mustard or mayonnaise, cut on the diagonal. There is no sauce doing structural work, no roll engineered for a load, no heat fusing anything together. What you taste is exactly the ham, exactly the cheese, and exactly the bread, in the ratio someone bothered to get right or did not. That nakedness is the whole identity. A ham and cheese is good only when its three ingredients are good and proportioned with some care, which is precisely why it functions as the baseline: it exposes everything.
The craft, such as it is, lives in restraint and proportion. The ham is sliced thin enough to fold and layered so the stack has give rather than reading as one dense slab. The cheese is usually a mild melting type, American or a soft Swiss or cheddar, picked to soften against the ham at room temperature rather than to assert itself. The bread is deliberately soft and close-crumbed so it compresses to the filling instead of fighting it, and it is the part most often gotten wrong, since stale or over-sturdy bread turns the whole thing to a chore. The mustard supplies the only sharp note and the mayonnaise the only fat, and a competent maker uses them to season rather than to drown. The sandwich is built in seconds at a counter, eaten without ceremony, and judged entirely on whether someone cared about three things that take no skill but do take attention.
The variations are small and honest, and each is one deliberate change to a fixed frame. Griddle it in butter and it becomes a hot, crisp-shelled thing closer to a grilled cheese with ham. Put it on a long roll with oil and shredded lettuce and it becomes the deli hero build. Spread the cheese as pimiento cheese and add a Southern country ham and it becomes its own regional reading. None of these pretend to be more than they are, and each deserves its own article rather than being crowded in here.