There are many discussions about so called altcoins. Is Ethereum an alternative coin? Is Ethereum even a coin? (This is a canonical question to gether answers and resources on that subject.)
Is Ethereum a fork of or based on Bitcoin code?
Ethereum is a brand new thing. Vitalik Butterin et al created it in 2014 from scratch. The only thing they used is the concept of blockchain initiated by Satoshi Nakamoto (we still don't know who he really is) and the experience of how Bitcoin works and fails.
See Ethereum Homestead documentation : http://ethdocs.org/en/latest/introduction/what-is-ethereum.html
In 2014, Ethereum founders Vitalik Buterin, Gavin Wood and Jeffrey Wilcke began work on a next-generation blockchain that had the ambitions to implement a general, fully trustless smart contract platform.