In 2013 the whitepaper for Ethereum was written by Vitalik Buterin. The whitepaper described an open-source, public, blockchain-based distributed computing platform that could run smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third-party interference.
Ethereum makes it possible for developers to build and deploy smart contracts, as well as issue their own cryptocurrency directly on the ethereum blockchain removing the need for the developers to create a new blockchain for their service. This does not just save the developers the time it takes to create a blockchain, it also gives them the security and decentralization of Ethereum which is not something inherent to all blockchains.
Smart contracts built on top of Ethereum make use of Ethereum's decentralized Turing-complete virtual machine, called the Ethereum Virtual Machine or EVM for short. This is the part of the protocol that actually executes the smart contracts/scripts. The EVM executes scripts using an international network of public nodes, ensuring the platform's censorship resistance.
Scripts or smart contracts on Ethereum are written using a new programming language built specifically for Ethereum called Solidity.
To incentivize people to run nodes and execute scripts, and to mitigate spam on the network, Ethereum has its own cryptocurrency, ether (ETH).
When you make an operation on Ethereum you need to pay an execution fee. The execution fee you pay is called “Gas” and is priced in ether. Gas measures how much work an action or set of actions takes to perform, the more computations the operation requires, the more Gas it will need. The amount of ether you pay per Gas is up to you, but remember that nodes prioritize requests depending on how much they pay.
With Ethereum comes benefits such as reliable uptime. If a company's server were to break, their service would stop working, whereas if any given node on Ethereum stopped working there would be many more around the globe keeping the service online. And the same goes for censorship, it's much easier for a malicious actor to shut down 1 centralized server than 100's or 1000's spread around the globe. This protects the service, making sure it’s always available for everyone in any part of the world.
So in short, Ethereum can be seen as a blockchain with a built-in programming language or like a consensus-based world computer that applications run on top of because they value the benefits offered by Ethereum over the ones offered by a normal server!