Ethereum (ETH) is the second-largest cryptocurrency and the leading smart-contract platform — a Layer 1 blockchain (its own base network) that works like a global, shared computer anyone can build on. Where Bitcoin is mainly digital money, Ethereum runs 'smart contracts' (self-executing programs), and those power most of crypto's real activity: DeFi (finance without banks), NFTs, stablecoins (dollar-pegged tokens) and thousands of apps. It switched from mining to proof-of-stake in 2022 (validators lock up ETH to secure the network instead of burning electricity), cutting its energy use by about 99%. ETH is both the fuel you pay to use the network ('gas') and an asset you can stake for rewards. Its supply isn't hard-capped, but a fee-burning mechanism can make it mildly deflationary when the network is busy.
Where it stands today: Ethereum is the settlement base of a huge ecosystem, but most everyday activity has moved to 'Layer 2s' (faster, cheaper networks built on top, like Arbitrum, Optimism and Base) that settle back to Ethereum. Recent upgrades pushed hard on this: Pectra (2025) improved staking and usability, and Fusaka (late 2025) multiplied the data space available to L2s, dropping their fees to a small fraction of what they were. Spot ETH ETFs now trade in the US and hold billions, and roughly a third of all ETH is staked. The next upgrade (Glamsterdam, expected later in 2026) aims to scale the base layer itself. In short, it's the most used and most developed smart-contract chain — but it has handed cheap transactions to its L2s and now lives on how much value flows through the whole stack.