Test networks, often referred to as testnets, are specialized environments used for testing blockchain applications and smart contracts without the risk of losing real cryptocurrency. They simulate the main network, allowing developers to experiment and debug their projects in a safe setting. Testnets use their own tokens, which can be obtained for free, making it easier to test functionalities without financial implications. These networks are crucial for developers in the blockchain space, as they help identify bugs and performance issues before launching on the main network, or mainnet. Popular testnets include Ropsten and Rinkeby, which are associated with the Ethereum blockchain, providing a platform for developers to ensure their applications work correctly.