What is a league and what is the difference between regular and ultimatum leagues?

Basically, leagues are like seasons, each league some sort of new gameplay mechanic gets introduced to the game. This league it was the ultimatum mechanic. Standard or how you called it regular league is basically a constant league where the content that is kept from the temporary leagues ends up. The current temp league usually has way more players so i would always start with that since theres no real downside. And after the league is over your character will be moved to the standard league anyway. Hope this helps if you have any more questions as away.

The ultimatum challenge leagues (standard, hardcore, and the SSF versions of both) are temporary leagues, separate from the permanent leagues. They include the ultimatum content, started april 16th, and will end in around two months at which point all characters from them will be migrated to the equivalent permanent leagues. Ultimatum content will likely be available in some form in permanent leagues after the merge, but won't be as accessible as it was in its challenge league. Challenge leagues also offer the ability to get exclusive cosmetic rewards for completing challenges that won't be available afterwards.

There is standard and league, each with hardcore, softcore and respective solo self found variants.

A league generally lasts 3 months, being a fresh start (like a new account almost, you keep the scion unlock, keep your trade perks you have to hit lvl 35 for, keep your hideout, and everything you bought for money and the cosmetic rewards you can win). It also includes a new mechanic, ultimatum this time, that might or might not get added to standard, often watered down and showing way less often.

When the league ends, everything gets moved over to standard: your character, items, currency, ... You can keep playing there, you just don't get the new league mechanics until the get added to the core (after the league).

Now, because this has been happening for years, the standard economy is totally different than league. There is so much currency from the years, that prices are generally higher. Most standard players are more collectors and rich people trying to create and buy perfect items, aka rich. There are items you can not make or find anymore, legacy items, because the stat or unique got nerfed, being too good.

The skills are the same, the classes are the same, the story, tree ... Just a bit more content and (in my opinion) better/beginner friendlier economy) in league, with the downside (or upside, I like it but it requires time not everyone has) of starting over every 3 months.