Mainnet vs Testnet
Plain English
Mainnet is the "real" blockchain where actual money and transactions happen. Testnet is a practice version where developers can test things without risking real money - like a sandbox or demo mode. Test coins have no real value.
Technical
Mainnet is the production blockchain network where real economic value operates. Testnets (Devnet, Testnet) are separate networks with identical protocols but no economic value, used for development and testing. Solana maintains Devnet for developers and Testnet for validator testing before mainnet deployment. Faucets provide free tokens on test networks.