Classic Zoids, heck the entire series of ZOIDS, takes place on a different planet called Zi, which like our Earth, is at the edge of the Milky Way Galaxy. However, Zi, as far as we know, is ridden with biological mecha ("giant robots") called zoids, created from a superior yet ancient civilization, the Ancient Zoidians.
Many newbie fans are often confused which is the actual first ZOIDS season, Toonami began airing the series with New Century Zero, which many have roughlyguessed to take place centuries after the events of Classic Zoids. But in fact, it's Chaotic Century that's the true beginning of the series.
In Chaotic Century, the Ancient Zoidians are long gone and basically unheard of by modern civilization,the Helic Republic and the Guygolos Empire, who are caught in a bitter war for years, and are using the zoids as their main weapon-of-choice. And then there's Van Flyheight, a young spelunker from the Wind Colony whom dreams of being a zoid pilot like his late father. It was one fateful day with a wild-goose chase from bandits when he uncovers the hidden past - the ruins of future friend and significant-other Fiona Alisi Lynette, and her organoid companion, whom Van names Zeke, after his late father's Command Wolf.
Then again, Van also gave Fiona her name, she didn't remember a clue but the name "Fiona". By the end of this "first-half" or "first season", however you callit, Van not only has found a zoid of his own, but becomes a fairly good pilot, meets many friends and enemies, and long story made short helps put a final stop to the war. Oh, and defeats the ancient evil known as Deathsaura, which could have destroyed the entire planet to bits and pieces.
Or did they? Enter Guardian Force, an approximate 3 or 4 year gap after Chaotic Century. Everyone is split apart to their own ways, but it's only a matterof fate that brings them back together when a rising megalomaniac named Hiltz tries to bring about the true Deathsaura. Not only does this work, this also tosses certain major antagonists over to fight the good fight, in order to save the planet once more from an inevitable apocalypse.
For more information on the series and the vast parallel storylines according to the model kits, check out the Wikipedia entry.