Invented by Vitalik Buterin in 2015, Ethereum is a decentralised platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third-party interference. Bitcoin is considered as a first generation blockchain whereas Ethereum is considered as a second generation blockchain; bringing a new generation of utility towards cryptocurrency.
A good way to understand the differences between Bitcoin and Ethereum is to recognise that each has their own goals and purpose. While Bitcoin was and still intends to be a digital currency based on the blockchain, Ethereum has sought to create an entire network within itself.