A Testnet is a separate blockchain network used for testing and development purposes. It allows developers to experiment with new features, applications, and smart contracts without risking real assets. Testnets simulate the main blockchain environment but use test tokens that have no real-world value. By using a Testnet, developers can identify bugs and improve their projects before launching them on the main network, known as the Mainnet. This process helps ensure that applications are secure and function as intended, ultimately benefiting users in the long run. Popular Testnets include those for cryptocurrencies like Bitcoin and Ethereum.