Ethereum's history
Ethereum was described in Vitalik Buterin's 2013 whitepaper, which can be found at: https://github.com/ethereum/wiki/wiki/White-Paper. He talked about the need of a scripting language that would run on top of Bitcoin, since he was involved in Bitcoin Magazine and he understood the limitations of the Bitcoin blockchain.
He saw an opportunity to create a platform that would run on decentralized technology to create new types of applications.
Not many believed in his vision, so he decided to create an entire new blockchain by himself with a small team that saw the potential in Vitalik's ideas. He founded the Ethereum Switzerland group and decided to run an Initial Coin Offering (ICO) in July 2014, where he sold Ether in exchange for Bitcoin, raising a total of about $18 million dollars.
He created the smart contracts technology, which is basically programs that run by themselves without requiring a trusted entity to execute them. They are always available, and they run without failure.
The fact that Ethereum provided a system that allows people to create their own applications on top of a blockchain is what made it successful. Before Ethereum, there was no simple way to create decentralized applications (dApps) in a decentralized platform. Bitcoin has a protocol to create simple applications using opcodes with a programming language called Script, but it's not capable of much since it is very low level and it's limited by the block size.