What is Ethereum?
by Héctor Ugarte
There are plenty sources of information on the web to answer this question. That is why instead of try to explain it explicitly, I will give some tips to understand this new platform quick and easy.
1. First, if you have no idea what is a cryptocurrency, I suggest you to start understanding what and how Bitcoin works. A very good book to do this is: Mastering Bitcoin by Andreas Antonopoulos (http://www.amazon.com/Mastering-Bitcoin-Unlocking-Digital-Cryptocurrencies/dp/1449374042).
2. Once you get the idea of bitcoin. Ethereum, is simple a Bitcoin 2.0 platform that takes the principle of achieve concensus without a central authority to also other use cases beyond money. In principle it also works very similar to Bitcoin, processing transactions, but in each transaction is possible to embeed code (Smart Contracts) that will let us build new Descentralized Applications(again, just another applications without needing a central authority to achieve concensus).
and Vitalik Buterin explaining it:
3. What are these others Descentralized Applications? In principle, every system that is working on a centralized manner can also be rewritten on a Descentralized way using Ethereum. But, surely the main idea of Ethereum is not that. Ethereum intention is not to overwrite all the existing systems in this new P2P blockchain way of thinking, but is to give a platform, that can handle Applications that really need Descentralization. The natural division right now is Financial and non Financial Applications. Very good usecases can be, Provenace of things in general, Electronic Vote, Crowfunding, Internet of Things, etc.
4. There are dozens of Other benefits of using a descentralized protocol like Bitcoin. Like keeping a permanent record of all transactions, where all the info is public, and for private information this can be simply encrypted. And one more important benefit, is that gives an Homogeneus framework for this Descentralized Applications, that can be a very benefitial point for Systems like Semantic Web based ones.
5. A very nice motivating video about Bitcoin: