The Reuben sandwich is a hot sandwich of layered meat, sauerkraut and Swiss cheese, with a dressing. These are grilled between slices of pumpernickel or rye. The meat is either corned beef or pastrami, and the dressing is either Russian or Thousand Island dressing. Several variants exist.
The correct sauce, by the way, is 3/5ths thousand island dressing and 2/5ths of the most profoundly brown and spicy mustard you can find.
On a related note: Why do sweet pickles exist; anywhere, at all? How is it not universally obvious that once dill pickles were invented there was absolutely nothing further to be done except to make them more fucking dill? The evil in this world man, the casual willingness to destroy beautiful things, it's fucking tragic.