Altcoins are any cryptocurrencies that were created after or based on Bitcoin technology. They are often modifications or improvements to some of Bitcoin's features, but may also have their own unique properties and protocols. Here are some popular altcoins and their brief description (as of my last updated data in 2021): Ethereum (ETH) Features: A platform for creating decentralized applications (DApps) and smart contracts. Key difference: Used to support ERC-20 tokens and other Ethereu...