Peanut butter and jam is the British reading of the build the Americans made a national habit, and what defines it here is that it is the rarer choice rather than the default. Peanut butter goes on soft white bread with strawberry or raspberry jam, and the jam is the answer to the peanut butter's central failing: where the bare peanut butter sandwich is dry, adhesive, and one-note savoury, the jam brings a sharp fruit acid that cuts the fat and a bright sweetness that lifts it. The defining fact is that the jam is not there to make the sandwich sweeter so much as to make the peanut butter bearable, an acid counterweight against a heavy, claggy filling that on its own has nothing to break it.
The craft is the order of the spreads and the control of two wet-and-fat layers that do not want to share a slice. The peanut butter is spread to the edges on one slice as a continuous fat seal, and the jam goes on the opposing slice, never on top of the peanut butter, so the dense layer keeps the jam's moisture off its own crumb and the two only meet when the sandwich is closed. Each is applied thin and measured: too much jam slides and bleeds pink into the bread, too much peanut butter brings back the cling the jam is meant to relieve, and the sandwich works only when neither dominates. Soft white bread is the convention because the filling supplies all the texture and any real crust would be the only thing a bite has to fight. The British version tends to a thinner, sharper hand with the jam than the American sweet-tooth build, which is part of why it reads as the same idea spoken plainer.
The variations stay inside the soft, sweet frame. A tarter jam, blackcurrant or damson, leans harder on the acid against the fat; banana in place of jam swaps the cut for soft sweetness; the wider sweet family of honey, banana, and chocolate spread keeps the same buttered-soft logic. Each deserves its own article rather than being crowded in here.