Ethereum is a peer to peer network that any developer can use to build decentralized apps. The decentralized apps can be anything, but the network on which they are built is optimized to carry out the rules that mechanically execute when certain conditions are met. Ethereum uses its decentralized public Blockchain to store, execute, and protect these contracts.
Every computer in its network downloads a small virtual machine that syncs with the Ethereum blockchain, and that remains available to execute contracts. As Ethereum is a computer network run by a community of users, the personal information of users is no longer stored on the central servers, which prevents it from getting sold, hacked, or disrupted.