Ethereum has many test networks where developers can deploy their dapps or the network can test fork changes before being moved to the mainnet. There are a handful of test networks, each of which have their own unique traits, making certain instances of testing better.
Ropsten - Proof of Work - Multi-client Testnet most resembling Mainnet - ~15s block times
Kovan - Proof of Authority - Parity only Testnet - ~4s block times
Rinkeby - Proof of Authority - Geth only Testnet - ~15s block times
Görli - Proof of Authority - Effort underway to establish multi-client PoA network