The code is how the rules under each game are sometimes described namely the 15 man code (union) or the 13 man code (league)
Things have changed in the recent past mainly due to the influence of your friends at Sky but historically Rugby Union was an amateur game played mainly in the south and south west of England wheras League was a professional game played mainly in Yorkshire and Lancashire. If you played Union and turned professional to play league you were then very much persona non grata as far as union was concerned.
More recently with the advent of Sky paying big money to televise, Union has also turned professional and players regullarly go from Union to league (and occasionally but less often in the other direction). They (Sky) have also turned Rugby League into a summer rather than winter sport.