What is a season?

I can find guides to a specific season, but having trouble getting info on what a “season” is / means. So far, what I’ve got is “season” is to “expansion” like “dialect” is to “language” + the “No Child Left Behind” Act of WoW gearing. I play mostly single player games and, these days, dabble in MMOs like a grand, casual, single-player experience when I have stretches of fairly empty time at work, so I’m always behind on terminology / systems.

Play history for context:
- On content progression through Wrath
- Break
- Last month of WoD, first patch of Legion as a super-casual old content farmer, then again with BfA
- This is my 4th week playing Midnight (2x iLvL ~270)

Questions:
- What is it?
- When did it start?
- How does it affect old content?
- What is it’s purpose? Both in theory and practice, if they are different
- Temp check on “seasons” being considered beneficial or detrimental to the players
- What is the difference between the release of a new season vs a new raid tier (or any X.# patch)?

Thank you in advance