Is Casper built on Ethereum?

I've been hearing a lot of buzz around the Casper upgrade lately in the blockchain and cryptocurrency community. But I'm a bit unclear about its underlying foundations. Could you please clarify - is Casper actually built on the Ethereum network? I've read that it's a proposed upgrade to Ethereum's consensus mechanism, but does that mean it's entirely integrated within the Ethereum ecosystem, or does it have a separate existence? It would be great to have a concise yet comprehensive explanation of the relationship between Casper and Ethereum.