The testnet(test network) is an alternative Waves blockchain, to be used for testing. Testnet WAVES coins are separate and distinct from actual WAVES, and are never supposed to have any value. This allows application developers or Waves testers to experiment without having to use real WAVES or worrying about breaking the main Waves chain.
Testnet is used a lot for developing purposes, assume you are building a new feature and its require you to send and receive WAVES and you don't have any real WAVES or you don't want to waste with mistakes, here you use test network (testnet).
A mainnet and a testnet are two separate networks that operate independently from each other.
|Purpose||Waves Functional blockchain||Waves Testing environment|
|Transactions||Waves real transactions stored on actual blockchain||Fake transactions|
|WAVES coins||Posses real value||No monetary value|
|Mining||Possible to earn rewards||Not possible|