Earth was the colony world of the thirteenth tribe of Kobol. It was a technologically backward world, and equivalent to the real-life Earth of that time.
Before reaching Earth, the thirteenth tribe laid down several colonies along the path to Earth, one being Terra, a much more technologically advanced world than Earth.
William Adama: What have I done? All the people I've sent to die, for what? For what?
Lee: For Earth.
Adama: There is no Earth. It's a frakking joke. There is no Earth.
Lee: Okay, Dad, listen to me. Listen to me! Pull it together.
--Lee trying to sober up a drunken William Adama.
Earth is the name for the planets seen as a "home".
The first Earth had long been destroyed before the Fleet could reach it. Radiation measurements of the water and ground showed that the planet had gone through a nuclear event long ago, and that it was once inhabited by humanoid Cylons and mechanical Cylons before this event. The planet was tainted with destruction, debris of fallen structures as far as the eye could see. The members of the Fleet declared the planet uninhabitable, and went back to their ships in search of another planet to call home.
Adama: Earth is a dream...one we've been chasing for a long time. We've earned it. This is Earth.
--Adama trying to convince Roslin that this the real Earth.
The second Earth was already inhabited by a vast amount of creatures and was covered by lush grass landscapes. Tribal human beings were the primal life forms on this planet, having the same DNA set up as the humans of the Fleet, providing a perfect match for breeding and thus the survival of the human race. It was here that Hera Agathon grew up and started the supposed maternal lineage for all humans in the following decades.
Many centuries later, in modern times, scientists had dug up the fossil of their so-called Mitochondrial Eve. The civilisation of that time was in a technologic boost, already capable of making simple robots.