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 middleman or counterparty risk.
To provide a trustless network you can connect to, deploy a piece of code responsible for defining your project's business rules in a way, the outcome of the code can be always predetermined. The state of the network can only change based on the rules known transparently, foreseeable by everybody. AKA decentralised state machine, Ethereum Virtual Machine (EVM).
Ethereum blockchain is like a Google cloud but instead of one company being in absolute power, multiple companies or yourself directly run simultaneously the same database in each server, communicating using the same protocol, consensus, based on Proof Of Work (PoW). Long story short, running the blockchain node must be an expensive operation to achieve security. Yes, not the smartest neither most scalable idea in my opinion.
Currently majority of the Ethereum infrastructure is run by the team developing the protocol, Ethereum foundation, so one could argue about the ratio of decentralization but that's another topic and it's absolutely understandable in such an early days of blockchain.