A random map is procedurally generated by a .js file calling upon a random-map-library. A scenario or skirmish map is created in the Scenario Editor, and is static, unable to be edited. I know that it is an option in the Scenario Editor to generate a random map, then save it as a Skirmish or Scenario map, and that a few people have written random map scripts to recreate scenario and skirmish maps as random maps.