Earth, or Terra, refers to the third planet of the Sol planetary system,which was the home planet for most of the original colonists of Pern.
The system is commonly referred to as "The Solar System". It has a G dwarf single star as its primary, and is orbited by numerous astronomical objects, most notably, 8 major planets, numerous minor, or dwarf, planets and several asteroid belts.
The third planet, Terra (called Earth by its inhabitants), has ideal conditions to support human life and as such, was the model for the type of planet that the Exploration and Evaluation Corps was searching.
The term "Terran" refers to things that originated on this third planet.