Harmony currently supports two networks: Harmony Mainnet and Pangaea (Harmony's community run testnet).
Mainnet is Harmony's production environment. It currently has four shards.
Pangaea is Harmony's testnet environment for dApp developers and partners. It is running the latest unstable version of the node software release, but not the nightly build. We do a rolling update of the testnet a few times per week. There is not a 100% guarantee that transactions on testnet will persist across the updates.