Bungie gave players a look at Destiny's new (and evidently pretty tough) Prison of Elders mode in a live stream earlier today. The short explanation is "yep, it's definitely a three-player horde mode" - but with all the flourishes and quirks you expect from high-level Destiny play.
The base-level Prison of Elders is rated for Level 28, and all of its challenges and modifiers are randomly selected. It's matchmaking-ready and you're free to play as many times a week as you want. If you have a pre-made team, you can also take a crack at the three challenges set at level 32, level 34, and level 35. Each one has its own name and themed modifiers and objectives, and new ones will rotate in weekly for each level slot.
Once you're in the match, you'll apparently have to clear five rounds of three waves each - I say "apparently" because the Bungie streamers, who were doing one of the tougher curated challenges, never made it past the fourth round.
Each round will apply a new mechanical modifier: you could get something beneficial like increased damage for primary weapons, or something a bit more troublesome, like boosted shields (yay) that don't ever recharge (boo). You'll also have to complete multiple critical objectives like defusing mines or taking out VIP targets. Thankfully, Prison of Elders warden Variks will occasionally offer gifts like heavy ammo or the explosive new Scorch Cannon to help you put down his unruly inmates.
Successfully clearing a match will reportedly reward players with a Treasure Key and access to a room filled with a random assortment of loot. Though, again, the stream didn't get that far. Even if it was a pretty anticlimactic end to the weekly House of Wolves preview streams, Prison of Elders does look pretty promising for casual pick-up players and hardcore teams alike.