Rinkeby is one of the most secure networks for testing your applications because it uses proof-of-authority (PoA) to securely generate blocks. In fact, it is so secure and stable that many use this network for prototypes, MVPs, and demos because developers know that their dApps will continue running without a problem on this chain.
It is identified by the ID 4 and it was created in 2017 by the Ethereum team to provide an alternative solution to developers that want to experiment with a different mining algorithm. This testnet is lightning fast: it generates blocks every 15 seconds consistently. The supply of Ether is controlled by puppeth so that people don't generate Ether with abusive mining practices.
The only supported client is Geth, although you can use it for most applications with MetaMask and Truffle.
