Typically, we speak of mainnet and testnet as the networks of Stacks. Most wallets are configured to mainnet by default—this is the production environment, the actual network that holds real STX tokens.
As the name suggests, testnet is a public network for testing. It's a separate network state that holds test tokens, which have no value.
For completeness we also mention devnet. This isn't "one" network, but how developers refer to ephemeral local networks used for testing via Clarinet. It is the same as testnet, but for local development.
Setting the network
Most Stacks.js functions accept a network parameter as a string or an optional last argument.