Ethereum is a biggest altcoin and the second crypto currency in crypto industry. Ethereum is a fork of Ethereum Classic. It has its own blockchain and based on ERC-20 protocol. Ethereum was build by Vitalik Buterin and other crypto enthusiasts in 2013 but started only in 2015.
The main role of Ethereum that it is a platform for decentralized finances. Many many new cryptocurrencies were build on its protocol ERC-20. For now Ethereum (ETH) is inflationary, but as team predict after verion 2.0 the count of ETH will decrese by burning and never new one will released.
Nowadays Ethereum has 116 millions coins in total supply and all of them are in circulating.
The main problem for Ethereum ecosystem is a very high gas fees. For one transaction it usually costs 20-30$ or more. And that's not good for retail investors.
Ethereum blockchain changed crypto industry by Proof of Work to Proof of Stake. If before you had to have videocards or any other special equipments, for now it is enough to have ETH and add liqudity to share staking rewards.