Official and pretty much easiest definition of Ethereum is next: Ethereum is a decentralized platform that runs smart contracts. Bitcoin was very first cryptocurrency ever, with the largest market capitalization, so many people are familiarize with concept of Bitcoin, so if you want to learn about Ethereum, advice is to compare these two cryptocurrencies. Ethereum is similar to Bitcoin where it comes to purpose. Both of these cryptocurrencies are used for financial transaction and as a store of value. One of the difference between these two is so called smart contract.
The main purpose of smart contracts are avoiding third side mediators. Ethereum is decentralized platform as we already said and it means that can decrease transaction costs and eliminate single point of failure. Also, with the smart contracts you don\'t need to rely on social media to store personal information. Another advantage of decentralized platform is that they are not using centralized servers so they can not be hacked or manipulated.
Advantage of decentralized platform is also fact that no single entity can overtake entire network. For example if Facebook or Twitter have been based on Ethereum there is no way that they would be criticized for politically motivated censorship, what recently happen. Decentralized funding and distribution of funds among several users are also tasks that Ethereum platform is doing well.
As we can see from the text above, smart contracts are making so many things easier to use and even cheaper, and it is bringing Ethereum network to the top of the market as well.