Testnet
In short: A testnet is an alternative blockchain network that developers use for testing. It closely mirrors the main network, but its coins have no real value.
A testnet is an alternative blockchain network that developers use for testing. It closely mirrors the main network, but its coins have no real value.
Why it matters
Mistakes on a live network can cost real money, and smart contracts are often hard to change once deployed. A testnet lets developers try new code, wallets and upgrades in realistic conditions first. Test coins are handed out free by services called faucets and exist only on the testnet, so they cannot be moved to the main network. Some testnets use different address formats to avoid mix-ups; Bitcoin testnet addresses, for example, can start with tb1 instead of bc1.
Example
A developer writes a new token contract. Instead of launching it straight away, they deploy it to an Ethereum testnet, request free test ether from a faucet to pay for gas, and try transfers, approvals and unusual edge cases. A bug turns up and is fixed at no cost. Only after the tests pass does the developer deploy the contract to mainnet, where every transaction uses real funds.