Altcoins, or alternative coins, refer to any cryptocurrency other than Bitcoin. These coins are built on the same decentralized blockchain technology as Bitcoin, but they have different features, use cases, and objectives. Some popular altcoins include Ethereum, Litecoin, Ripple, and Bitcoin Cash. Altcoins often offer faster transaction times, lower fees, and different mining algorithms compared to Bitcoin. Many altcoins are created to improve upon the original Bitcoin design, while others have entirely different use cases such as privacy, smart contracts, and supply chain management. The altcoin market is highly dynamic and diverse, with new projects constantly emerging and gaining popularity.