Diablo 3's end-game content is all about rifts. Normal rifts, greater rifts, empowered rifts. There's a lot of rifts. One more will be added to the gear-farming pile once the game's next patch arrives, with the introduction of "Challenge Rifts".
Rather than just upping the health and damage on mobs, painting them a different colour and tossing players once more into the inter-dimensional breach, Challenge Rifts will present a "static" test for players, drawn from previously-completed Greater Rifts.
If that sounds confusing, allow Blizzard's "Nevalistis" to explain:
To create a Challenge Rift, we pull a Greater Rift run directly from a player’s account. Once we have that snapshot, we make that exact character, including their items, paragon levels, skills, and gear available for everyone to play.
That's not all. Pretty much every aspect of the rift will be replicated, from pylons to enemies. The idea is to beat the original completion time, after which you'll receive a "satchel filled with bounty and crafting materials and some Blood Shards".
The announcement goes on to say that Challenge Rifts are meant to "break the mould" of Diablo 3's randomisation-heavy gameplay.
If anything, they should give you some way of measuring your own abilities, outside comparisons with those on your friend list.
First Look: Challenge Rifts [Blizzard]