Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, censorship, fraud or third party interference.
These apps run on a custom built blockchain, an enormously powerful shared global infrastructure that can move value around and represent the ownership of property. This enables developers to create markets, store registries of debts or promises, move funds in accordance with instructions given long in the past (like a will or a futures contract) and many other things that have not been invented yet, all without a middle man or counterparty risk.
On traditional server architectures, every application has to set up its own servers that run their own code in isolated silos, making sharing of data hard.
If a single app is compromised or goes offline, many users and other apps are affected.
On a blockchain, anyone can set up a node that replicates the necessary data for all nodes to reach an agreement and be compensated by users and app developers.
This allows user data to remain private and apps to be decentralized like the Internet was supposed to work.