Elder Rifts are randomly generated short dungeons. In order to complete it, you need to kill a certain amount of monsters and defeat the Rift Guardian, all within 15 minute time limit. You can access Elder Rifts via Elder Rift Entrance in Westmarch. Inside Elder Rifts you can find various Shrines providing temporary buffs. Also enemies in there can drop Experience Globes. There's also a chance to spawn a unique vendor when you kill the Rift Guardian.
Enhancing Elder Rifts
You can use special items - Rare and Legendary Crests to enhance an Elder Rift. Doing so will add a modifier affecting both players and monsters, this modifier can be either positive or negative. Each player in party can use up to 3 Crests on a single Elder Rift. The number of modifiers the Rift will have is based on the maximum amount of Crests used by one player.
Completing an Elder Rift on time will award you Gold, XP and Items. Additionally, if someone in your party used Crests to enhance this Rift, you will receive some Fading Embers and a chance to find Runes or Legendary Gems. The amount of Fading Embers received is based on the total number of Crests used by all players.