Rivendell is an Elven settlement in Middle Earth ruled by Elrond Half-Elven.
Rivendell (or Imladris, in the Sindarin tongue) was founded by Elrond some time early in the Second Age, making the settlement nearly 5000 years old at the start of The Hobbit.
It is located in a “hidden valley” in the foothills of the Misty Mountains, on the edge of gorge where the River Bruinen cuts its way through the foothills.
Rivendell plays an important role in both The Hobbit and The Lord of the Rings. Bilbo and Company stop there during the first leg of their journey to Lonely Mountain in The Hobbit, where Elrond provides valuable insight into their map of the mountain and the origins of their swords.
During The Lord of the Rings, Rivendell holds an important role as a “safe house”, where the Ring and Ringbearer may rest without worry. I also holds host to the “Council of Elrond”, one of the most important scenes in The Lord of the Rings where the wise argue over (and eventually decide) what should be done with the One Ring.
Rivendell is also referred to as “The Last Homely House” in both The Hobbit and The Lord of the Rings. Bilbo calls it in The Fellowship of the Ring “a perfect house, whether you like food or sleep or story-telling or singing, or just sitting and thinking best, or a pleasant mixture of them all” (FotR pg. 237).