From wow.incgamers.com

Summary

• Levels: 66-70
• Players: 5
• Layout: 3 Wings
  • Old Hillsbrad - Escape from Durnholde Keep (Level 66+ mobs, 5 player groups)
  • Opening of the Dark Portal (Level 66+ mobs, 5 player groups)
  • Battle of Mount Hyjal (25 man raid, access gained through reputation with the Keepers of Time)
• Main Enemies: The Infinite Dragonflight, a flight of dragons that are attempting to change history by changing large world events.
• Location: Tanaris

The Caverns of Time currently has 3 wings with a fourth one that may or may not be opened come release of the expansion. The first wing players enter is Old Hillsbrad where they rescue Thrall from Durnholde Keep, the second wing players help Medivh open the Dark Portal and in the third wing players re-enact the Battle of Mount Hyjal where the Legion was defeated.

The main enemy during all these instances is the Infinite Dragonflight who are attempting to alter history by not allowing Thrall escape, Medivh to open the Dark Portal and keep the mortal races from defeating the Legion.

Attunement for the Battle of Mount Hyjal Raid

In order for players to gain access to the Battle of Mount Hyjal they will need to complete some prerequisites. First players will need to defeat Kael'Thas in Tempest Keep and Lady Vashj in Coilfang Reservoir both instances also have their own attunement requirements. After the 2 are defeated you will have access to the Battle of Mount Hyjal.

4th instance portal

Explanation in following interesting Tigole's post:

We have a ton of content we want to add to CoT. It's one of our favorite areas. Originally, the Battleground for the Burning Crusade was going to take place in Hellfire and be accessible through CoT. But as we got into the design of the battleground mechanics, we found that Netherstorm made more conceptual sense for what we had designed so we moved the BG to Netherstorm. That doesn't mean we won't revisit our original idea, however. We're still very excited about all of the possibilities CoT has to offer.