So what is ascendancy? Can someone explain

Okay so I only recently started this game. I was fortunate enough to have a gotten a lot of exalted currencies when I was leveling so I was able to build strong characters. I did not research about this game so now i hear about ascendancy and from the looks of it, it has new classes and a new act

With that said, I assume the Talisman League is going to be done and everything will be transferred to the standard league (similar to d2). So can I expect this to happen?

Yes, you get transferred to standard.

It is not new classes, it's specialties within classes.
There will not be a new act, just a new dungeon (labyrinth)

On March 3, the Talisman leagues end and all characters and items in them move to the permanent leagues. There's a pretty reasonable likelihood of the talisman mechanics, stone circles, etc being incorporated into the game permanently, but this hasn't yet been confirmed.

On March 4, Ascendancy is launched. This adds new areas to the game (some randomly spawned in existing zones, and one nonrandom one in Act 3). Completing quests in these zones unlocks ascendancy classes, which are specialisations of your base class with powerful new passives, and also opens up some new crafting possibilities. There will be new temporary leagues launched alongside Ascendancy, but we don't yet know anything about them. Expect them to be much less in-depth than Talisman because we're already getting so much new content.