So with Taken King, it includes the two Year 1 DLC packs and integrates everything into the same Director screen.
- The Vanilla story will take you through each of the planets (including the moon) and is centered on the Vex. The end game is the Vault of Glass raid where you fight Atheon.
- After you meet the Hive on the Moon, you unlock the first misisons in The Dark Below which focuses on Crota and his return to reconquer the moon. The end game is Crota's End raid where you fight Crota in his sword realm.
- As you travel to Venus, you need help understanding the Vex and visit The Reef, a social hub where the Awoken who live in the asteroid belt. There you begin missions related to the House of Wolves, a faction of Fallen lead by Skolas who have escaped and are rampaging across the inner solar system. The end game activity is the Prison of Elders, which recently held the House of Wolves and Skolas, and will again.
- At a certain point (not totally sure when but I guess after vanilla & tdb parts finish), you unlock Taken King missions which begin on a moon of Mars. Taken King adds many quest lines that further extend story and quests from Year 1, such as more things related to the Reef. Oryx has arrived and has Taken enemies across the solar system and you must stop him. His Dreadnaught hangs in orbit near Saturn and you will eventually take the fight to him, just as you learn that the Cabal have as well.
It's a really impressive job they've done. You're in for a lot of great fun.