Season 1 was the first set of Dice Throne characters to be released. These are the individual sets that each include 2 characters.
Season 2 was the second set of Dice Throne characters to be released. These are the individual sets that each include 2 characters.
I was so pumped when Marvel Dice Throne went up on Kickstarter. They really do a great job of capturing the feel of each character. Also check out the Box Sets section because the individual sets of two characters can be hard to find.
The box sets are a great entry point into the game. You can get all of season 1 or two, or the Marvel set. Newly released is the X-Men set, but it's not widely available yet.