Hey BoomDaddyS,
So I'll use Forza since I'm most familiar with it to explain the difference.
DLC or Downloadable Content is extra content that can be purchased to enhance a game's experience by adding more feature or extending the storyline. For Forza, this can be Car Packs or Expansions. They can be bought separately as you wish, however, to purchase all of it would be costly.
A Season Pass grants you all or almost all (up to the developers) of the DLC of a game to a certain date. This allows you to get all the DLC as it is released for a discount. Saving will vary based on the game.
For Forza,
They have what is known as a car pass (like a season pass for other games that offer say maps).
Here is one for Forza 4 via the website:
"Get 42 extra cars at one low price! The Forza Horizon 4 Car Pass will deliver two new cars to your game every week over a 21-week period, beginning October 2, 2018. If you start playing the game after this period has begun, all previously released cars included with the Car Pass will be available for your game garage. All cars included in the Car Pass grant you a one-time only, free addition of each car to your Forza Horizon 4 garage, with no additional in-game credit cost. The Forza Horizon 4 Car Pass is included with Forza Horizon 4 Deluxe and Ultimate Editions. "
The Season Pass for games vary depending on what the developer wants to include. For Call of Duty, it may be access to all the released skins and maps pack for the entirety of the game on release.
Hope this helps!,
Best,
Nozzle :)