TL;DR: I will explain Ethereum and its smart contracts in five levels for different audiences.

Child

Ethereum is like a giant computer that runs on the internet.

Imagine playing a board game with this computer ensuring everybody follows the rules.

It can also store your items safely so that no one can cheat.

All the digital robots obey the rules without anyone changing them.

While Bitcoin is like digital coins you can use, Ethereum is like a computer that can do more things, not just handle money.

Teen

Ethereum works similarly to a massive internet-based computer.

You can also use it to send money, create digital pictures, or build online games.

You can play your games there but no one controls the network and not even the person who created it.

The special part is smart contracts. They are like legal contracts you sign in real life but live on the Internet instead of a folder.

Once you start a smart contract, it does its job automatically and nobody can change it.

Bitcoin lets you send and receive money, while Ethereum helps you create all kinds s of applications using smart contracts.

College Student

Ethereum is a decentralized platform that lets programmers make applications using smart contracts.

Smart contracts execute on the Ethereum blockchain.

These contracts follow specific rules and execute transactions without the need to trust a middleman, like a bank or a lawyer.

Ethereum also has its currency, Ether that you can use to pay for running applications or sending transactions.

You can create your tokens using smart contracts mimicking real-world goods like USD or GOLD.

Since the system is decentralized, the network is not governed by a single entity.

Because of this, it is secure and transparent, and you can be sure that a contract will operate just as intended once it is in effect.

Graduate Student

Ethereum is a decentralized platform that uses smart contracts to execute decentralized apps using blockchain technology.

Smart contracts on Ethereum allow for trustless agreements. Once deployed, these contracts execute transactions autonomously based on the conditions defined in their code.

This approach lowers expenses while boosting security by eliminating the need for third parties.

To execute these contracts, you must pay a cost named "gas" with Ether, the native currency of Ethereum.

The flexibility of Ethereum's programmable blockchain has sparked an ecosystem of decentralized finance, non-fungible tokens, and more.

Expert

Ethereum is a decentralized blockchain platform that is Turing-complete and allows smart contracts to be executed.

These self-executing contracts rely on consensus across a global, trustless network, ensuring immutability and censorship resistance.

Ethereum's proof-of-stake consensus mechanism secures the network by allowing validators to propose and validate new blocks based on the amount of Ether they stake.

Developers write smart contracts in Solidity, compile them to bytecode, and then deploy them to the Ethereum Virtual Machine (EVM).

Ethereum's flexibility enables complex decentralized applications, decentralized finance protocols, and non-fungible tokens.

Ethereum is evolving toward scalability and sustainability through its roadmap for Ethereum 2.0, incorporating sharding to enhance throughput and reduce energy consumption.

Ethereum will be the backbone of Web3.