Testnet

Testnet is a public Bitcoin-protocol network that runs Bitcoin's consensus rules with intentionally worthless coins. It exists for developers and operators to test wallets, transactions, and node configurations without risking real mainnet BTC.

Testnet currently runs as testnet4 (the third reset, activated 2024). Earlier versions (testnet, testnet3) accumulated enough abandoned hash power and abuse that they were periodically retired and replaced with fresh chains. The current testnet4 has:

• Free coins from faucets. Various sites give out small amounts of testnet BTC to anyone who asks. They're effectively worthless.
• A separate magic genesis block. Testnet chains are not the same as mainnet; transactions and addresses are not interoperable.
• A modified difficulty rule. If 20 minutes pass without a block, difficulty drops to the minimum so the chain doesn't stall during low-activity periods. Mainnet has no such escape hatch.

The drawbacks of testnet:

• It's chaotic. Difficulty swings wildly, blocks can come in bursts or droughts, abusive actors spam it. You can't rely on testnet behavior to predict mainnet behavior.
• Test BTC has occasionally become traded. Despite being officially worthless, scammers periodically try to sell testnet BTC to confused buyers, or use it for proof-of-funds scams.
• Not appropriate for security-sensitive testing. If you want to test a high-stakes upgrade under realistic adversarial conditions, Signet is usually a better choice.

For most casual development - "does my wallet work, can I broadcast a transaction, what does fee handling look like" - testnet is fine. For anything more serious, look at Signet or a private regtest network.