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 Ethereum-based standards. Litecoin (LTC)

Features: Designed as "silver to Bitcoin's gold", has a shorter block time and a different hashing algorithm. Key difference: Faster transaction confirmation compared to Bitcoin. Ripple (XRP)

Features: Focuses on providing solutions for financial institutions for international transfers. Key difference: Not based on PoW or PoS, but uses a consensus mechanism. Cardano (ADA)

Features: A smart contract platform with an emphasis on security and formal verification. Key difference: Developed with an academic approach and scientific rigor. Chainlink (LINK)

Features: A decentralized oracle network that allows smart contracts to access external data. Key difference: Connects the blockchain to the real world. Polkadot (DOT)

Features: A platform for connecting different blockchains into a single ecosystem. Key difference: Helps different blockchains transfer messages and values directly. Binance Coin (BNB)

Features: Created by the Binance cryptocurrency exchange, used for commission discounts when trading on Binance. Key difference: Is a token of a major cryptocurrency exchange. These are just a few examples of altcoins out of thousands that exist on the market. It is important to understand that each altcoin has its own specifics, purpose and purpose. In addition, the cryptocurrency market is changing rapidly, so new altcoins may appear, and old ones may lose their relevance.