The East sometimes called the Eastlands refers to the large region Middle-earth and includes the lands of Rhun, the east is also used to refer to the armies of Mordor contrasting to the West of Gondor and Rohan.
In the time of the Two Trees of Valinor and the First Age of Arda, it was the location of the inland Sea of Helcar, the mountain range of Orocarni (Mountains of the East), Cuiviénen, the birthplace of Elves, and Hildórien, the birthplace of Men in the first year of the sun. After the destruction of Beleriand, the shape of the region changed considerably becoming more of dried land consisting of scattered woodlands and lowlands. The Sea of Helcar disappeared, possibly shrinking into the smaller Sea of Rhûn, Cuiviénen merged with the land and was no longer along the coast, Hildórien still bordered the East Sea to its east; however, both of these realms were probably no longer inhabited as they once were. The Orocarni Mountains still remained intact and unbroken.
The Elves and Dwarves (in the mountains) were the first to inhabit the East and still continued to do so even after many Elves left on the Great Journey to Valinor. Later during the First Age, Men inhabited the land and quickly spread into the west but only the tribes of the Edain moved into the Beleriand escaping the Dark Powers, while the earlier Easterling tribes stayed and continued to serve them.
Throughout the Second and Third ages, the region still continued to be inhabited by the Easterlings who completely dominated the area and still served the Dark Powers. The Easterlings also sometimes refer to their land as the Land of the Rising Sun. Dwarves probably still lived in the Orocarni mountain range as well as other men and creatures. The Blue Wizards - Pallando and Alatar were said to have gone out there in wagons, but never returned. However, Tolkien never expanded much on this area.