Vitalik Buterin proposed Ethereum in 2013 and saw it through to execution in 2014-15. Unlike the Bitcoin network, which was specifically created for Bitcoin transactions, Ethereum is a platform for decentralized apps (or Dapps) that uses smart contracts.
The Ethereum platform is completely decentralized because it is run by thousands of independent computers. When a programme is deployed to the Ethereum network, these computers, known as nodes, ensure that it runs as designed. Ethereum enables people to connect directly with one another without the need for a centralized authority to control things. It is a network of computers that work together to form a powerful, decentralized supercomputer.
Here is a great resource for learning more about the Ethereum blockchain APIs.