Testnet
A separate blockchain network used for testing and experimentation that mirrors the functionality of its corresponding mainnet but uses tokens with no real monetary value. Testnets allow developers to deploy, test, and debug smart contracts in a realistic environment without risking real funds. Users can obtain free testnet tokens from 'faucets.'
“Ethereum's Sepolia testnet lets developers test their dApps before deploying to mainnet. You can get free Sepolia ETH from a faucet to pay for gas on the test network.”
Mainnet
The live, production version of a blockchain network where real transactions with real economic value take place. A mainnet launch represents the moment a blockchain goes from development and testing to public operation. Assets on mainnet have real market value, unlike testnet tokens which are worthless.
Smart Contract
Self-executing programs stored on a blockchain that automatically enforce the terms of an agreement when predetermined conditions are met. Smart contracts enable trustless transactions without intermediaries because the code, once deployed, executes exactly as written and cannot be altered (unless specifically designed to be upgradeable). They form the foundation of DeFi, NFTs, DAOs, and virtually all dApps.
dApp (Decentralized Application)
An application that runs on a decentralized blockchain network rather than being hosted on centralized servers. dApps use smart contracts for their backend logic while providing user-friendly front-end interfaces. They are censorship-resistant and typically open-source, though their performance depends on the underlying blockchain's capabilities.