The world created by Eru and the Valar for elves and other races to dwell in. Eru composed a great music, and used the flame imperishable to fashion the Valar to play his music. Arda was created by the music.
Elves' return to Arda brought magic back to the sphere, and a great many things are awakening and changing. Prophecy from Mandos suggests that we have probably triggered the end times, leading up to Dagor Dagorath.
Places on Arda - description and detail
Arda's crystal sphere raw - compiled by the Emperor.
Biblical Arda raw - compiled by the Emperor.