Ethereum is a decentralized software platform, which offers the possibility to build and run decentralized applications based on blockchain
technology. It is much more than a simple cryptocurrency
Applications created in Ethereum (also called smart contracts) can not be modified, censored or controlled by a third party or a central server. It is a peer-to-peer system, where users make transactions with their smart contacts by using a runtime environment called “Ethereum Virtual Machine”
For running this technology, you have to download (or create) an Ethereum client. It will automatically connect to Ethereum network and you will be able to create smart contracts, to perform transactions and to mine new blocks.