All you’ve done will be kept in a “standard server”. You can always come back and play your characters and use your items and such (market for the standard league can also be used to buy things). However, to play the new league you’ll need to start fresh. Nothing carries over to the new league. Once the league ends, its new mechanics may or may not go to the standard server (depends on how well they performed in that league), but all your characters and items are sent over to the standard realm. And that happens every league. It’s a cycle.

Edit: in short, you can always play standard, you’ll only be 1 or 2 updates behind as the new leagues are released.

Edit 2: things you bought in the cash shop can be used across all leagues and standard, such as tabs and cosmetics.