Altcoins are all cryptocurrencies other than Bitcoin. The term is a combination of "alternative" and "coin." Altcoins can vary widely in terms of technology, purpose, and value. Some well-known examples of altcoins include Ethereum (ETH), Ripple (XRP), Litecoin (LTC), and Cardano (ADA). These coins often introduce different features or improvements over Bitcoin, such as faster transaction times, different consensus mechanisms, or enhanced privacy.