The game has it set up that you either "Find a job" or you "find a profession". Rabbit holes are the ones like the criminal career where your Sim goes in and you don't see them until it's time to get off work or you cancel work for whatever reason. A profession is an interactive job like the ones that came with Ambitions, Generations or Showtime. These are the ones that are xp based and require you doing things to advance. Examples of rabbit holes: Criminal, Music, Law Enforcement. Examples of professions: Acrobat, Firefighter, Ghost Hunter, DayCare.
Also note that the Medical career is considered a rabbit hole even though it has some of the interactive features that would normally mark a profession.