A hominid (Homo sapiens) sentient species native to Planet Earth. While Terrans and Earthers are the same thing, the term Terran usually describes all humans that originated from Earth and is used as a term to describe the race, while the word Earther is exclusively used for individuals who are actually born on Earth.
(Example: Richard Stahl is a Terran and an Earther as he was born on Earth and is a human of the Terran species. Eric Olafson, born on Nilfeheim is technically a Terran but he is not an Earther.)
(Picture shows Terran Slaves from a Slave Dealers Catalog on Rafcor III)